[DX][gkg][indexing] More granular control over workspace indexing and exclusions

Problem to Solve

It is expected behavior for gitalisk to recursively track and separate deeply nested repositories with a .git folder into entities that will be indexed by gkg, independently. This functionality makes sense for the GDK, as it's a repo of repos.

However, this comes with the downside of obviously indexing everything in a workspace, even external dependencies like git@2.5.1 that you are just testing.

Proposed Solution

As gitalisk is behaving as expected here, we might want to instead allow "Indexing Profiles" that we can add to the gkg manifest, to exclude entire subtree from indexing, to compliment our tracking of .gitignore.