[feature] Design mechanism to define default settings

[nemesifier]

Is your feature request related to a problem? Please describe.

It's not uncommon that a specific setting needs to be added to the project settings, due to a change in a dependency or in a module.
This creates a bit of friction when maintaining the installers.

Describe the solution you'd like

I am wondering if we can find a solution for this which avoids the need of constantly modifying the settings.

If in each module we could specify a set of default settings that have to be automatically added to the project settings, we may make maintenance easier, we may also make upgrading custom deployments easier.

Describe alternatives you've considered
N/A.

Additional context
E.g.:

• [deps] Update django-allauth requirement from ~=0.54.0 to ~=64.1.0 openwisp-users#403
• [chores] Added allauth.account.middleware.AccountMiddleware ansible-openwisp2#487