roachprod: centralized api bootstrap - cluster sync and registration

Problem: Roachprod users currently manage clusters directly using cloud provider credentials on their local machines. This approach spreads cloud credentials widely, makes cluster tracking compute intensive, and lacks centralized visibility into cluster state. When multiple users create clusters independently, there's a high risk for collision, and no efficient way to manage clusters.

Solution: Build a centralized REST API that provides cluster synchronization and registration capabilities.

The first API iteration should expose endpoints to:
- Sync clusters from cloud providers (AWS, GCP, Azure, IBM) into a central database
- List all clusters with filtering capabilities
- Register new clusters created externally
- Update the clusters registry when changes occur
- Remove clusters from the registry when deleted

This creates a single source of truth for cluster state and enables teams to see all active clusters without direct cloud provider access.

Jira issue: CRDB-56179

Epic CRDB-49123