[DPE-3180] Share secrets to host application

[MiaAltieri]

Issue

1. Mongos charm needs to be notified of updated username/password
2. URI construction in mongos should consider connections made via the Unix Domain Socket

Future PR

Most of the work for DPE-3180 is done on the mongos charm. But the mongos charm relies on the libraries hosted by mongodb charm to make the necessary changes future work:

1. updates libraries
2. shares connection info to the application hosting the subordinate mongos charm
3. integration tests verifying its functiondd