Optimizations for MultiTenantAuthenticationSchemeProvider

[MarkCiliaVincenti]

No description provided.

[AndrewTriesToCode]

Hi @MarkCiliaVincenti this is an interesting PR, thank you. I will be reviewing but just a heads up I have to be somewhat conservative in that I support older version of .NET and C# and also only take dependency on core .NET packages. I really like the use of Lock here but I can't yet use preview language settings for the overall project and I can't take a dependency on the 'Backport` library.

I will ultimately rotate these updates in as .NET 9 releases and do something like:

#if NET9_0_OR_GREATER
global using Lock = System.Threading.Lock;
#else
global using Lock = System.Object;
#endif

[MarkCiliaVincenti]

That would work but would limit what you can do, basically not call any of the methods offered by the new lock class.