Add support for project.toml

[matthewmcnew]

Add support for the Cloud Native Buildpacks Project descriptor RFC.

[jromero]

Adding a use case I recently heard:

As a developer, I would like to run the same build configuration locally via pack as kpack. This includes the setting of build time environment variables and buildpacks to execute.

The project.toml file would be a good way to share that configuration across CNB platforms.

[genevieve]

This is a great idea! Would this look like a new field on the Image or Build resource that points to a url for a specified project.toml that is separate from the main source (for example the application's git repository)? Our usecase is that the project.toml for most of our applications is identical (i.e. it contains the same list of files to exclude) and instead of copy+pasting that file everywhere, it would be great if there was a configuration param to the build resource that lets us point at a different url for a shared project descriptor file.

[matthewmcnew]

I'm going to keep this issue open as a tracking issue for all the remaining project descriptor fields not implemented in #690.