Add a source package that can be used to connect to the roslyn build server

[jjonescz]

This package will be used by the SDK CLI to communicate with the build server (so dotnet run file.cs can skip MSBuild and "run csc directly" instead when possible - note that it cannot actually start csc.exe because process start overhead would negate any optimizations from skipping MSBuild). See https://github.com/dotnet/sdk/issues/48011.

The set of files from the package is also used by the Replay tool to dogfood them in this repo (and to verify it is a coherent set of files).