Experiment with generating code from proto files using Roslyn source generator

`Grpc.Tools` uses msbuild to integrate protoc into .NET projects. Using protoc has pros and cons:

* Pros: Works today, handles all versions of Protobuf, well tested, maintained by Protobuf team
* Cons: Native files aren't always portable, large NuGet package size, hard to change, external dependency

We could use the approach of `Grpc.Tools` to integrate into the .NET project file, but use a source generator to parse and produce generated files instead of protoc.

I think this would be a significant amount of work to handle all the vageries of parsing proto files, handling dependencies, imports, proto2, proto3, etc. If work is done here it should start with a small spike.

Start by reviewing how Grpc.Tools integrates protoc into .NET project files, then assess a small source-generator spike for parsing proto files and producing generated files. Consider dependencies, imports, proto2, and proto3; done means documenting whether this approach is feasible and what limitations remain.

Written by the indexing model from the issue text.