Epic CRDB-11087

This is a pre-req to solving #72045

To add a feature to "execute some SQL statements before a node starts draining" (#72045), we must have a draining process to start with. The start and start-single-node commands provide that, but mt start-sql doesn't.

Describe the solution you'd like

We want to provide a Drain RPC and a shutdown sequence to the mt start-sql server process that is using a common code path with the one used for regular DB nodes.