feat(backend): serve keys from payment pointer url

[omertoast]

Changes proposed in this pull request

• add paymentPointerUrl field to clients table. This is temporary as I don't want to break services that use clients, if you have a better way to link clients to payment pointers I'm open to it.
• add getClientByPaymentPointerUrl method to client service.
• add getPaymentPointerKeys method to paymentPointerRoutes to serve key set.
• add /jwks.json route.

Context

This is one of the first tasks for refactoring the Key Directory from v1 to v2. As the first step, I'm implementing /jkws.json relative path to the payment pointer URL to serve the client key set linked to the payment pointer.

TODO

• [x] Link clients and payment pointers
• [x] Allow getting clients using payment pointers
• [x] Extract JWKS from the key set
• [x] Serve key set from /jwks.json endpoint.

Checklist

• [ ] Related issues linked using fixes #number
• [ ] Tests added/updated
• [ ] Documentation added
• [ ] Make sure that all checks pass

[adrianhopebailie]

LGTM but I suggest either @mkurapov , @sabineschaller or @wilsonianb have a quick glance over

[omertoast]

I was able to create a client, add key to client, and get the keys via /jwks.json path successfully. For this PR, just should get Brandon's suggested change in there and good to go

I started a branch for moving clients functions and table into the paymentPointer. Should be ready end of the day.