feature request: Dot project extension support

[mlieberman85]

Note: I know dot project is just a POC right now, but I wanted to throw a few ideas out there as I'm playing around with some tooling.

Based on discussions, I assume there will probably be two sets of fields for .project:

• Part of the spec
• Additional data that doesn't conflict with the spec that other tools can use and .project tooling

I think there should be something added to the spec to define how other tools could extend .project.

My two cents is just specify certain reserved file names e.g. project.yaml or like repo.yaml or something and then anything not specified could go into its own yaml files under .project/.