Add unauthenticated concurrency limit

It's useful to treat unauthenticated requests different from authenticated ones. Let's add a section in the concurrency limiter config that allows this, and limit the request accordingly.

[[concurrency]]
rpc = "/gitaly.CommitService/CountDivergingCommits"
max_per_repo = 10
max_queue_wait = "1m"
max_queue_size = 10

[[concurrency.unauthenticated]]
max_per_repo = 2
max_queue_wait = "1m"

fixes: https://gitlab.com/gitlab-org/gitaly/-/issues/6941