Add Flavours

[OldMetalmind]

It is required to configure the Flavors in the app to easily add more run ways of the app, such as, running FogosAdmin.

Notes:

• Avoid adding libraries dependencies
• https://docs.flutter.dev/deployment/flavors

[OldMetalmind]

One wish also is to have a flavour of the app that does not use any trackers. So we can add it to other open-source stores and that safeguards the privacy of the users.

Related: https://github.com/FogosPT/fogosmobile/issues/170