[Payment] Experiment with payment through AppSync

[heitorlessa]

As per discussion on #87, there's value in bringing Payment into GraphQL Schema and possibly integrate with Stripe directly via HTTP data source.

We'd also need custom mutations and fine-grained authorization to secure certain transactions such as refund.

Initial steps to make this happen:

• [ ] Create Payment type in GraphQL
• [ ] Create custom mutation to collect and refund payments
• [ ] Create custom resolver to call Stripe
• [ ] Apply fine-grained authorization to allow IAM operations only
• [ ] Merge collect payment with confirm booking
• [ ] Update refund payment to use GraphQL API instead