Implement "full flow, transfer out" integration test for FiatConnect APIs

[jophish]

This ticket is for implementing the "full flow, transfer out" tests for FiatConnect, described here; this includes both the webhook and polling cases. A starter repo is available here.

In the future we may open source these tests, so partners can use them to test their own implementations. (Deciding whether to do this is outside of the scope of this issue. May reinforce the perception that Valora handles QA...)

AC:

• Full flow, transfer out tests are written and added to the repo linked above.
• Includes test for polling and test for webhook
• different sets of mock data / schemas / config can be supplied by a json file so we can test different providers

[cajubelt]

Silas started this here https://github.com/fiatconnect/fiatconnect-validate

[cajubelt]

plan is here: https://github.com/fiatconnect/docs/blob/main/e2e-plan.md