[DPE-5215] feat(libs): Correct use of data_interfaces

[Gu1nness]

Issue

• In the Mongos k8s charm, we saw that the data which is expected to be stored as secrets are shown in plain sight.
• A thorough investigation and the help of @juditnovak lead us to see that we were not using the data_interfaces library the correct way, breaking the expected flow, which lead to this issue.

Solution

This library was not using the data interface library the right way. The provider should not write in the databag before the database_requested event which is fired when the requirer asks for the DB and write its name + the requested-secrets list in the databag.

This commit hence:

• Defines a handler for the database_requested event
• Protects the call to relation_changed events by checking first if the database field is in the relation databag, which can happen only after the database_requested event has run.

[Gu1nness]

I have logged a ticket to do the implementation you are suggesting @juditnovak but I think this is out of scope for this ticket and I don't have some additional bandwidth to handle this during this pulse. This will be reviewed, scoped, and tackled as soon as we have some time.