[Booking] Create booking reference before triggering state machine

[heitorlessa]

Currently, process_booking mutation in AppSync currently triggers a Step Functions async, and reserve-booking Lambda function is the first called to create a booking reservation.

Idea here is to improve front-end experience by creating a booking reference earlier, and to let the customer know what the next steps are with a better design.

• [ ] Bring pipeline resolver back to process_mutation
• [ ] Bring reserve-booking as an AppSync function to be triggered before processBookingSfn function
• [ ] Move booking reference creation from confirm-booking to reserve-booking
• [ ] Update front-end to reflect changes