[MongoDB] Enable Add-on and include auth support

[AngelosF]

Enabling the Add-On: The persistence service MongoDB is NOT visible/available in PaperUI for for installation on openHAB 2.4 Snapshot (and I believe the same is true also for OH 2.3)

Enhancing the Add-On: Currently, the binding configuration does not have authentication support. It would be nice to have them added (not very important since not many users actually use MongoDB for persistence and even fewer will need auth for the connection to the DB

Potentially a Bug exists with the restoreOnStartup of Contact Items. Check here: https://community.openhab.org/t/mongodb-persistence-with-openhab-2/31949/19 Note: This may have been miss-configuration on behalf of the user... not sure

Refs:

1. https://community.openhab.org/t/using-mongodb-persistence-to-store-openhab-item-updates-on-mongodb-atlas-database/57307
2. https://community.openhab.org/t/mongodb-persistence-with-openhab-2/31949
3. https://github.com/openhab/openhab-distro/issues/487 (when I was young with github and was messing around in the wrong repos 📦

...unfortunately, I don't feel confident enough to perform: https://www.openhab.org/docs/developer/development/compatibilitylayer.html#how-to-add-a-successfully-tested-1-x-add-on-to-the-distribution

Ps: @kaikreuzer What about using JDBC (https://www.openhab.org/addons/persistence/jdbc/) to support MongoDB also? Is this possible (to extend JDBC functionality to cover MongoDB) or preferable ?