Is your feature request related to a problem? Please describe.

We rely on roachtest to test the stability and correctness of multi-tenant deployments. However, roachprod does not offer built-in functionality to orchestrate tenants (it is completely oblivious to their existence). roachtest has a few test helpers that make it reasonable straightforward to start a tenant, but leaves much to be desired in terms of UX.

Describe the solution you'd like

roachprod should support tenants:

• roachprod start-tenant [...]
• support for the expanders that we have for the host cluster such as {store}, etc., and in particular, a fixed convention for ports assigned to tenants
• roachtest abstraction over the tenant that makes using them as productive (easy creation and lifecycle monitoring, automatic artifacts collection, etc, Start/Stop/Monitor) as using a pure host cluster.

Describe alternatives you've considered

Additional context

cc @srosenberg
cc @cucaroach since this came out of our recent session talking through your roachtest #77481

Epic CRDB-10428
Jira issue: CRDB-14151