Snapshot recovery with offloaded data

**Is your feature request related to a problem? Please describe.**
1. Backup and restore an existing cluster to/from the cloud
2. Much cheaper DR without geo replication.
3. Bootstrap pulsar clusters quickly for dev/test from the cloud.

**Describe the solution you'd like**

1. create-offload-snapshot manually to create a recoverable snapshot in offload storage.
2. set-offload-snapshot-interval to create recoverable points in offload storage.

**Describe alternatives you've considered**

Geo replication works for DR but too expensive.

**Additional context**

Don't need snapshot on bookkeepers if it makes the impl simpler.

The issue names no files or tests. Start by locating Pulsar’s snapshot and offload-storage entry points, then determine how cluster recovery should use manually created or interval-based recoverable points. Done means backup and restore work from offload storage through both requested controls, with bookkeeper snapshots optional as stated.

Written by the indexing model from the issue text.