feat(backend): add services for managing RS grants

[wilsonianb]

Changes proposed in this pull request

• add auth server and grant services to backend
• export and fix auth types

Context

• progress towards Request grant at other Open Payments server(s) #583
  TODO: perform grant request on quote creation

Checklist

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

[sabineschaller]

add auth server and grant services to backend

Is this actually moving auth into backend? I don't want to merge the two. The auth server should be plugable.

[wilsonianb]

Is this actually moving auth into backend?

No, this is adding tables and services to the backend to manage:

• Open Payments auth servers (not including its own) at which it has grants to make client requests
• its grants

[wilsonianb]

If we want, this pr could wait for:

• Adding AS types & grant requests support to open-payments client #727

and then be updated to fully resolve:

• Request grant at other Open Payments server(s) #583

(and import types from open-payments instead of auth)

[wilsonianb]

Converting to draft for 👆

[wilsonianb]

• Replaced by feat(backend): request grant to query incoming payment receiver  #779