[DPE-2995] Create mongos user

[MiaAltieri]

Issue

related mongos suboridinate charm doesn't have its own user

Solution

Create a user for related monogs subordinate charm

Future PRs

1. implement relation on mongos suboridinate charm
2. enable mongos charm to change its requested database / roles
3. share URI with hosting charm of mongos
4. add int tests once both charms are up to date on charmhub

Testing

# deploy shards + config server
juju add-model test-0
cd ~/mongodb-operator
charmcraft pack
juju deploy ./*charm --config role="config-server" config-server
juju deploy ./*charm --config role="shard" shard

cd ~/mongos-operator
cp ~/mongodb-operator/lib/charms/mongodb/v0/config_server_interface.py lib/charms/mongodb/v0/config_server_interface.py
cp ~/mongodb-operator/lib/charms/mongodb/v1/helpers.py lib/charms/mongodb/v1/helpers.py
tox -e build
juju deploy ./*charm
cd ~/mongos-operator/tests/integration/application
juju deploy ./*charm


# relate
juju integrate mongos application
juju integrate config-server:config-server shard:sharding
juju integrate config-server:cluster mongos:cluster


juju ssh mongos/0
sudo charmed-mongodb.mongosh mongodb://relation-5:DOVn0liQ2taeIaLyL3yii8pYO4fsjoRW@%2Fvar%2Fsnap%2Fcharmed-mongodb%2Fcommon%2Fvar%2Fmongodb-27018.sock