Add Stream boilerplate source-generator and attribute

[Jozkee]

Forked from https://github.com/dotnet/runtimelab/tree/standalone-template.

• Adds the System.IO.StreamSourceGeneration project containing the generator and the [GenerateStreamBoilerplate] attribute.

• Adds test verifying generation of a variety of streams that implement one or more members that can be used to hint to the generator what it should do:

  • StreamThatImplementsNothing
  • StreamThatImplementsReadAsyncByte
  • StreamThatImplementsReadAsyncMemory
  • StreamThatImplementsReadByte
  • StreamThatImplementsReadSpan
  • StreamThatImplementsSeek
  • StreamThatImplementsWriteAsyncByte
  • StreamThatImplementsWriteAsyncMemory
  • StreamThatImplementsWriteByte
  • StreamThatImplementsWriteSpan

This PR does not add, but is intended as a foundation for adding analyzer rules that hint the user about potential improvements on their Stream implementation. e.g: "Prefer overriding Span/Memory overloads instead of calling byte[] ones."

• Adds several diagnostics (without fixers).

[Jozkee]

Ping @stephentoub @bartonjs, this is ready for re-review.

[Jozkee]

Ping @stephentoub.

I'm planning on adding tests to ensure we emit the proper diagnostics, functional tests checking the behavior of the generated code already exists. In the meantime, this PR could benefit from a code review.