Implement Client and Tenant Management APIs

Add APIs to create, update, and manage SSH signer clients (e.g., HPC environments) and their configurations under each tenant.
This should allow tenants to onboard new compute systems via the Custos UI or API, defining host, port, CA policies, and rotation rules dynamically instead of static configurations.

The issue names no files, tests, or implementation entry points. Start by locating the existing Custos UI or API paths for tenant and SSH signer configuration, then clarify the contract for host, port, CA policies, and rotation rules. Done means tenants can create, update, and manage compute-system configurations dynamically.

Written by the indexing model from the issue text.