Allow for passing a project-to-build via contents over stdin

[baronfel]

Summary

Users should be able to create a text model of a project, then pipe it to stdin and have MSBuild use that input stream to create a Project(Instance) for the build request.

Background and Motivation

This would make it easier to simulate multi-project builds and might reduce the need for us to do more extreme work.

Proposed Feature

echo "<Project></Project>" | dotnet build should work.

Alternative Designs

No response