feat: Enable custom prefixes for checkpoint savers (#125)

[vishal-bala]

Closes #125

This PR implements customizable key prefixes for checkpoint saving (largely aligned to how they were implemented for stores in #101).

[bsbodden]

I think we need change the the isolation tests (test_custom_checkpoint_prefix_isolation_sync) to:

• Not start each tests with a fresh Redis container
• Not have the Savers created sequentially, but instead do it concurrently in the same instance

Something along the lines of:

 with RedisSaver.from_conn_string(redis_url, checkpoint_prefix="app1_cp") as saver1:
          saver1.setup()
          with RedisSaver.from_conn_string(redis_url, checkpoint_prefix="app2_cp") as saver2:
              saver2.setup()
              # Both savers should be able to use their own indexes independently

[bsbodden]

LGTM! Thanks!