Add option to git_repository to provide build file / content

[limdor]

This is just a first draft from the discussion we had with @aiuto in https://groups.google.com/g/bazel-dev/c/sfeRE3Ucj7E just to see what the CI says about it. Before this can be proposed to be merged, I see several topics to tackle:

• When new_git_repository rule should be removed?
• Can it be deprecated in a way that is not just text in the documentation?
• Should this PR already replace the internal references to new_git_repository to git_repository or should be done when removing the new_git_repository?

Probably we should also create an issue to track it.

[aiuto]

I'll merge this.