[FEATURE REQUEST] Add an explicit customer registration process independent from bookings

[matt-wirtz]

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

Currently the process of customer registration is handled implicitly while creating bookings. This prevents use cases where non trip related purchases like a monthly subscription should be performed. It also prevents pre-booking user registration. Some registration processes take some time - even days - on the TO side to perform all checking. During this time the booking needs to remain in pending state until all checking has been successfully done. That makes it more complicated on MP side to handle and communicate to the user.
Additionally it makes GDPR processes like "where is my data stored" much more complicated.

Urgency

Major (the TOMP API works as advertised but this is really necessary to implement), because non trip related purchases are quite common. Many TOs offer subscriptions that allow e.g. for better time rates when using their service.

Additional context

Describe the solution you'd like

Add an explicit process of customer registration. That is then the basis for adding the capability for subscriptions and other non trip related purchases.

Describe alternatives you've considered

Possible Implementation