feat(stripe): integrate pay-by-card

[beniaminmunteanu]

This PR implements stripe integration, by utilising stripe 'advanced integration' options:
PaymentIntents for registering the payment
and PaymentElements for embedding a form

The implementation serves as reference for integrating "pay by card" functionality in interledger-pay.

Following flow is implemented:

(delimited by the horizontal red line, the final webhooks are not implemented here. These need to be implemented in the Wallet backend- testnet repo)

Note: Implementation is not final.
TBD In order to close #41 and #42 :

• The Pay by card option should only be visible based on env. variables, as interledger-pay deployment should not be affected. The changes in this PR should only be applied on a separate image that is used when reverse proxy responds to wallet address request with the pay-by-card enabled interledger-pay image
• PR is functionality oriented, and frontend design needs work.
• The receiver wallet address should be pre-filled from url.

P.S.
For local testing, change sameSite to 'lax'

interledger-pay/app/session.ts

Line 9 in 2f2c47f

sameSite: 'none',