roachprod: centralized api - service discovery

Problem: The current system (ab)uses a managed DNS zone with SRV records for cluster discovery. This approach is very heavy for the DNS provider, and doesn't support keeping track of roachprod service info (e.g. secure/insecure flags).

Solution: Replace the managed DNS zone approach with a cluster registry in the centralized API service:
- Store service registration data (cluster services, ports, protocols) in the centralized database
- Automatically register services with prom-helper-service when clusters are created or updated
- Provide API endpoints to query available services and their locations

This enables proper service discovery patterns, reduces manual monitoring configuration, and provides a more scalable approach to managing cluster networking information.

Jira issue: CRDB-56182

Epic CRDB-49123