Draft: Implement PackObjectsHookStream
Part of gitlab-com/gl-infra/scalability#1126
This adds the implementation of PackObjectsHookStream. This RPC uses StreamRPC and because of that this MR depends on !3644 (closed). Besides the change in protocol, PackObjectsHookStream does the same thing as PackObjectsHook.
This MR introduces a new feature flag pack_objects_hook_stream which determines whether gitaly-hooks will call PackObjectsHook or PackObjectsHookStream.
Edited by Jacob Vosmaer