Add glId, glUser, glRepo, glProtocol to PackObjectsRequest

In order gain visibility into which user or repository is responsible for concurrent pack objects processes, !4596 (merged) adds functionality that will log these metrics. However, the PackObjectsHookWithSidechannel does not get user_id and username in the context. The reason is because it's gitaly-hooks that makes an RPC call.

user_id and username get injected into the context by workhorse and gitlab-shell, so PostUploadPackWithSidechannel and SSHUploadPackWithSidechannel have access to these values through gRPC metadata. But, when gitaly-hooks gets called, we don't pass these values in.

This MR adds functionality to pass these values through the GITALY_HOOKS_PAYLOAD env var to gitaly-hooks, which then can unmarshal this payload and pass in user_id, username, and protocol to the PackObjectsHookWithSidechannel RPC call.