Enable specifying git_clone_depth to aws_codebuild_project

[jebbens]

It would be useful to specify git_clone_depth for each module to speed up the DOWNLOAD_SOURCE phase of a build.

[lorengordon]

We don't have a need of that ourselves, as DOWNLOAD_SOURCE is plenty fast for our projects and probably the last thing in our builds we'd ever optimize. However, we're fine exposing more options and PRs are welcome! Feel free to use the prior PR adding the python_runtime as an example.