MessagePack for C# v4: Round4 - #2290

This round resolves the netstandard2.0 -> net10 compatibility problem that was the main concern from the previous round.
Rewrites MessagePackFormatterFactory, and implements nearly all formatters.
With this, the core is now mostly finalized.

• DLLs compiled against netstandard2.0 now run on net10 as well
  • Previously this only went as far as throwing an exception with a dedicated message. This has been moved forward: at the Serialize/Deserialize root we now detect whether the graph contains legacy formatters, and if so, pass compatible buffer implementations as IWriteBuffer/IReadBuffer, so everything just works
  • The analyzer that raised an error when net10 was not included in the targets has been removed — it was unnatural and placed too much burden on third parties
  • I consider the v4 compatibility problem solved with this
  • The performance penalty of the compatible tier is around +4–7%
    • Given the overall speedup, this is an acceptable range — even the compatible tier is still overwhelmingly faster than the others
• IMessagePackFormatterFactory -> MessagePackFormatterFactory
  • The API now provides MessagePackFormatterFactory.Default, .DefaultAot, .DotNetOptimized, and .DotNetOptimizedAot as the built-in factories, plus .Combine() to return composed ones
  • DefaultAot is Default with GenericFormatterFactory removed, and is therefore not [RequiresDynamicCode]
  • DotNetOptimized uses formats more optimized than the default — for example, binary for Guid. Since DotNetOptimized can also read data in the Default format, you can migrate from Default to DotNetOptimized later
    • It does not use any dangerous mappings, so it is safe and can be used with untrusted data.
• Collection formatters implemented to meet security requirements
  • Depth checks during serialization/deserialization
  • HashDoS resistance using SipHash-1-3
  • Validation of malicious length headers
• The output format of the Default formatters maintains 100% compatibility with v3
  • OldSpec is automatically compatible for reading only; writing is not supported