Add payment integrations to builder

Extracted from that forum discussion

A payment integration should meet these criteria natively (without the need for code or an external automation tool):

• authentication with payment gateway must be native
• user rights must be synchronised with the payment conditions set
• the application builder must be able to provide access in several modes: freemium, lifetime licence, monthly/quarterly/annual subscription, usage, etc.
• payment plans must be able to evolve without changing for current users (pricing change)
• invoice generation must be automated
• payment plans must be different for different roles
• in the case of a dual-role application: we can imagine that access is paid for role 1, free for role 2, and that role 1 pays according to the number of role 2s he invites to his team
  (workspace system)
• we could imagine pricing that depends on the number of records attached to a user