Rest api

[gamebeaker]

Description of the feature

Implement rest api to allow using a other database than the local sqlite.
This would allow to separate the database from moneymanagerex and enable other uses.

Reasons:
The reason i got this idea is that i have a nextcloud server and searched for a money manager app on nextcloud and found nothing Budget App - any interest?.
If moneymanagerex-"server" is an app on the nextcloud store it would reach new people.
If rest api is supported it doesn't matter which database is in the backend (Postgres, Mariadb, MySQL etc.).

I am aware of web-money-manager-ex and that you can sync via your own cloud (dropbox etc.) if you use android-money-manager-ex.