Add LoggerSetup for AOT-compatible custom logger registration (#7902)

[ArinF1]

Summary

Resolves #7902

Adds LoggerSetup to enable programmatic, AOT-compatible custom logger
registration — an alternative to HOCON-based logger config, which relies
on Type.GetType() and is not safe in AOT scenarios.

Changes

• Akka.Event.LoggerSetup — a Setup subclass holding an ordered
  list of logger Props, built with LoggerSetup.Create(params Props[])
  or LoggerSetup.Create(IEnumerable<Props>). It mirrors the shape of
  ExtensionsSetup.
• LoggerSetup.Formatter — an optional ILogMessageFormatter, set
  through LoggerSetup.Create(ILogMessageFormatter, params Props[]). The
  same AOT problem that applies to a custom logger type also applies to a
  third-party log message formatter — for example Akka.Logger.Serilog's
  SerilogLogMessageFormatter or an NLog equivalent — since
  akka.logger-formatter also resolves by Type.GetType().
• LoggingBus.StartDefaultLoggers — starts the LoggerSetup loggers
  and the HOCON akka.loggers list together (see "Maintainer changes"
  below).

Usage

var setup = ActorSystemSetup.Create(
    LoggerSetup.Create(new SerilogLogMessageFormatter(), Props.Create<MyCustomLogger>()));

var system = ActorSystem.Create("MySystem", setup);

A LoggerSetup is additive, not a replacement: the default
akka.loggers = [DefaultLogger] keeps printing to standard output
alongside MyCustomLogger above, unless you also set akka.loggers = [].
Formatter works the same way for akka.logger-formatter — it wins when
set, otherwise HOCON resolves the formatter as it does today.

Maintainer changes

I made the following changes before merge, based on review feedback:

• Additive, not replacing. LoggerSetup used to skip akka.loggers
  entirely. That silently dropped any HOCON-configured logger, including
  Akka.Hosting's LoggerFactoryLogger (written by AddLoggerFactory()).
  StartDefaultLoggers now starts the LoggerSetup loggers and the HOCON
  list together. A logger type named by both starts once — the
  LoggerSetup is checked first and wins, the same order
  ActorSystemImpl.LoadExtensions uses for ExtensionsSetup versus
  akka.extensions. One side effect: a type repeated more than once
  within akka.loggers itself now also starts once, not once per
  occurrence.
• Shrank the API to the ExtensionsSetup shape. Dropped
  LoggerRegistration and LoggerSetupBuilder — LoggerSetup now just
  holds IReadOnlyList<Props>, built with Create(params Props[]) /
  Create(IEnumerable<Props>). Props.Create<T>() already carries the
  trimming annotations Prop-based construction needs, so the extra
  wrapper types and their own [DynamicallyAccessedMembers] plumbing
  added no value. LoggingBus now has a single AddLogger(system, Props, ...), and the HOCON path builds Props.Create(type) itself.
• Fixed a generic-logger name bug this shrink surfaced: a generic
  logger's Type.Name carries a backtick and an arity digit (e.g.
  MyLogger`1) that isn't a legal actor-path character;
  CreateLoggerName now swaps it out.
• Added LoggerSetup.Formatter, so a custom ILogMessageFormatter
  (Serilog, NLog, etc.) has the same code-based escape hatch a custom
  logger type already gets. Settings checks it before falling back to
  akka.logger-formatter, and the NotBuiltIn message for that setting
  now mentions it as the code alternative.
• Merged dev into the branch to pick up the AOT and logger-resolution
  work that landed since this PR opened, and resolved the merge conflict
  in LoggingBus.cs using the additive design above.
• Replaced the spec file with a single generic CapturingLogger<TMarker>
  (a static per-closed-type instance counter, no polling loops) and specs
  covering: a HOCON logger plus a LoggerSetup logger that also repeats
  the HOCON type (proves the additive behavior and the dedup); a
  LoggerSetup logger and formatter both working with
  Akka.DynamicTypeLoading switched off; and the LoggerSetup formatter
  winning over akka.logger-formatter when both are set.
• Updated CoreAPISpec.ApproveCore.DotNet.verified.txt for the final
  surface; left CoreAPISpec.ApproveCore.Net.verified.txt matching dev,
  since this environment can't build or verify that target.

Verified: dotnet build -warnaserror on Akka.csproj; the Logger specs
3 times each (no flakes); the Logger, Event, and AkkaFeaturesSpec specs
in Akka.Tests (235 passed); Akka.API.Tests (24 passed); and the trim
analyzer, which reports the same warnings as dev with none new from
LoggerSetup.cs, LoggingBus.cs, or Settings.cs.

Follow-up

Not part of this PR, but worth tracking separately:

• Akka.Hosting's LoggerConfigBuilder.AddLogger<T> writes a type name
  into HOCON today. It should route through LoggerSetup instead, now
  that one exists.
• Only one LoggerSetup exists per system — like any Setup, a second
  one passed to ActorSystemSetup.And replaces the first rather than
  merging with it. That is consistent with ExtensionsSetup today, but
  it means two libraries that both want to register a logger through
  LoggerSetup will need to coordinate.

[ArinF1]

n

[Aaronontheweb]

Thanks for this, @ArinF1 — LoggerSetup is a nice addition for AOT users. I pushed a few follow-up commits to your branch to get it ready for merge:

• Made LoggerSetup additive with akka.loggers instead of replacing it, so it doesn't silently drop a HOCON-configured logger (like Akka.Hosting's LoggerFactoryLogger). A logger type named in both now starts once.
• Changed the factory delegate to take ExtendedActorSystem instead of the internal ActorSystemImpl.
• Added [DynamicallyAccessedMembers] annotations so the trimmer keeps what Props.Create needs.
• Merged dev in and added a few more specs covering the additive behavior and the dynamic-type-loading-off path.

Everything's green: build, the logger/event tests, the AOT feature specs, and the API approval tests. Details are in the PR description under "Maintainer changes." Nice work, and thanks for your patience while this sat in the queue.

[Aaronontheweb]

LGTM - thank you!