Add possibility to predefine pgAdmin user roles in configuration

[ViliusS]

Describe the solution you'd like

We already have a possibility to predefine server list and define pgAdmin preferences through JSON files. It would be great to have similar possibility for pgAdmin user roles too. It would be very useful for container type deployments in OAuth2 multi-user environments were one needs to forbid managing server list, or accessing Maintenance Tab.

Describe alternatives you've considered

I tried to come up with manual scripts to modify pgadmin.db internal database during container deployment, but this is not prone for future DB structure changes.

Additional context

Functionality should at least should allow to modify current User role which is automatically assigned to every OAuth2 user.