Add configuration several shards

What does this Merge Request do?

Adds how we can configure several shards in Gitaly with one Praefect node.

Merge Request checklist

• [ ] Tests added for new functionality. If not, please raise Issue to follow-up.
• [ ] This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• [ ] gdk doctor test added, if needed.