Mock Stripe API

[dunkOnIT]

This PR introduces a function which allows the Stripe API function to be stubbed, and will disable net_connect by default (it is still enabled for certain tests -for example, ensuring that we are providing the Stripe API with a well-formed payment_intent creation request).

The intention is to allow us to deliberately call the Stripe[1] API when it makes sense to, and mock endpoints in all other cases

Note: I almost certainly haven't stubbed the API everywhere it needs to be stubbed - I'm letting the CI suite run to help me identify where stubbing needs to happen.

[1] and other external services, but just Stripe for now