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

### 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 "" | dotnet build` should work.

### Alternative Designs

_No response_