[POC] Integrate the Datahub application in GeoNetwork web app

[jahow]

This is a proof-of-concept for the proposal https://github.com/geonetwork/core-geonetwork/issues/8021

Work-in-progress, not intended to be merged

To use:

• Once GeoNetwork is running, go to System Settings and enable the Datahub; give it a custom configuration if needed The Datahub can be accessed on http://localhost:8080/geonetwork/srv/datahub
• The Datahub app can be also be enabled for subportals; a different configuration can be provided In that case it can be accessed on http://localhost:8080/geonetwork/{subportal}/datahub