mgr/dashboard: select any object gateway on local cluster.

mgr/dashboard: select any object gateway on local cluster.

Dashboard backend settings:
- Refactoring: now accepting more than 1 type of value.
- RGW_API_ACCESS_KEY & RGW_API_SECRET_KEY accept string (backward compatibility: legacy behavior) as well as dictionary of strings for connecting multiple daemons.
- Ease of use: deprecated: mgr/dashboard/RGW_API_USER_ID: not useful anymore (kept for backward compatibility).

UI/UX:
- Created context component (to be shown only on rgw-related routes) for selecting operating daemon.
- Daemon selector only shown if there is more than 1 daemon running on a local cluster (to reduce cognitive load).

Fixes: https://tracker.ceph.com/issues/47375
Signed-off-by: Alfonso Martínez <almartin@redhat.com>