Accept Path option for GetArchive RPC

Updates the GetArchive RPC to accept a Path option.

This option will be used when calling git archive for archiving a directory in a repository.

CE Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/24704

gitaly-proto MR: gitaly-proto!281 (merged)

TODO

• Update vendored gitalypb and gitaly-proto gem when related gitaly-proto MR gets released.