[OpAMP] Clean up internal directory and namespace

[stevejgordon]

Fixes #3609

Changes

• Move all public types out of the "Internal" directory and update namespaces accordingly.
• Move internal messages into a specific "Messages" directory and update namespace accordingly.
• Update using directives for all dependant types.
• Update unshipped public API

One other

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.43%. Comparing base (57c7f48) to head (5349ca1).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3612   +/-   ##
=======================================
  Coverage   71.43%   71.43%           
=======================================
  Files         442      442           
  Lines       17522    17522           
=======================================
  Hits        12517    12517           
  Misses       5005     5005           

Flag	Coverage Δ
unittests-OpAmp.Client	78.05% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...nTelemetry.OpAmp.Client/Internal/FrameProcessor.cs	83.33% <ø> (ø)
...nt/Internal/Messages/AgentIdentificationMessage.cs	0.00% <ø> (ø)
...mp.Client/Internal/Messages/CapabilitiesMessage.cs	0.00% <ø> (ø)
...y.OpAmp.Client/Internal/Messages/CommandMessage.cs	0.00% <ø> (ø)
...ent/Internal/Messages/ConnectionSettingsMessage.cs	0.00% <ø> (ø)
...ent/Internal/Messages/CustomCapabilitiesMessage.cs	0.00% <ø> (ø)
...p.Client/Internal/Messages/CustomMessageMessage.cs	100.00% <ø> (ø)
...p.Client/Internal/Messages/ErrorResponseMessage.cs	0.00% <ø> (ø)
...try.OpAmp.Client/Internal/Messages/FlagsMessage.cs	0.00% <ø> (ø)
...ient/Internal/Messages/PackagesAvailableMessage.cs	0.00% <ø> (ø)
... and 3 more

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