Add pygit2 support to git fileserver backend

This pull request adds pygit2 support to the git fileserver backend, alongside the existing GitPython support. It establishes minimum requirements of 0.19.0 for both pygit2 and libgit2.

Additionally, a new master configuration file parameter ``gitfs_provider`` is added to manually specify which provider should be used. If this parameter is not present, pygit2 will be preferred over GitPython.

I've included language in these commits that targets this feature for the Helium release. @thatch45, let me know if this sounds good.