Create Open Payments client library

[wilsonianb]

The backend has some existing Open Payments client functionality for resolving quote/outgoing payment receivers:

• feat(backend): add Open Payments client service #595

This functionality should exist in its own library that is generally usable for any Open Payments client to perform and validate Open Payments requests and responses.

It can include the following pending functionality:

• Self-provisioned keys #606
• Request grant at other Open Payments server(s) #583
• Generate httpsig for internal OP calls #587

This could potentially also export generated Open Payments types (as well as OpenAPI request/response validators):

• Use generated Open Payments OpenAPI types #662

[mkurapov]

Remaining feature-parity issues:

• (open-payments): get quote  #809
• (open-payments): create quote #810
• (open-payments): continue grant  #811
• (open-payments): cancel grant #812
• (open-payments): rotate token  #813
• (open-payments): revoke token #814
• (open-payments): complete incoming payment #815
• (open-payments): list incoming payments #816
• (open-payments): list outgoing payments  #817
• (open-payment): use signature creation code from http-signature-utils #818

[mkurapov]

Feature parity with the Open Payments API complete, closing this issue. Only outstanding related issues to the open-payments client are

• Use generated Open Payments OpenAPI types #662
• Document open-payments package open-payments#248