Set up Roslyn Public API Analyzer

[adamreeve]

Set up the Roslyn Public API analyzer which should detect changes that break the API or ABI.

We need per-target public API files due to having some methods that use the Half, DateOnly and TimeOnly types that are only available for targets >= net6.

I've had to disable some warnings about overloaded methods that have optional arguments. We can't change those now that they've been shipped.

Any local changes can be automatically added to the unshipped API txt file with:

dotnet restore
dotnet format analyzers --diagnostics=RS0016 --no-restore

(Others might not need the separate restore and --no-restore argument but I had to do it that way to work around a bug)

Then when we make a release we should manually move anything in the unshipped files over to the shipped files.

There are more details in https://github.com/dotnet/roslyn-analyzers/blob/main/src/PublicApiAnalyzers/PublicApiAnalyzers.Help.md

[marcin-krystianc]

I naively thought that we could avoid the burden of tools like this and, with enough care during manual reviews, prevent the API/ABI from being broken. However, as we've recently seen, this is not the case. Having automation to help maintain API/ABI stability is essential.

Well done 🏆