[DPE-2562] POC Copy secrets from config server to shard

[MiaAltieri]

Issue

Sharded components should have the same keyfile as the config server, and we want to use the same operator username/password for shards as the config server

Solution

Pass secrets to shards when they join a relation with the config server and have shards listen for this event and act appropriately

Limitations

This does not consider what happens when the user changes the password on the config server, that should be handled in a separate PR

Testing

As a POC there are not tests included in this PR. Testing was performed by hand by hand:

juju deploy ./*charm --config role="config-server" config-server-one -n3
juju deploy ./*charm --config role="shard" shard-one -n3
juju integrate config-server-one:config-server shard-one:sharding

# verify can only relate a shard to a single config server
juju integrate config-server-two:config-server shard-one:sharding
ERROR cannot add relation "config-server-two:config-server shard:sharding": establishing a new relation for shard:sharding would exceed its maximum relation limit of 1 (quota limit exceeded)

# verify same password
juju run config-server-one/0 get-password
juju run shard-one/0 get-password

# verify equal key files 
juju ssh config-server-one/0
sudo cat /var/snap/charmed-mongodb/current/etc/mongod/keyFile
juju ssh shard-one/0
sudo cat /var/snap/charmed-mongodb/current/etc/mongod/keyFile