Support pagination with `after` and `before` both specified

[wilsonianb]

The relay pagination spec supports both after and before being specified together:
https://relay.dev/graphql/connections.htm#sec-Pagination-algorithm

Rafiki currently ignores before if after is specified
https://github.com/interledger/rafiki/blob/3d30ce755b4218a777ec77f408be5d15dfa0e687/packages/backend/src/shared/baseModel.ts#L64-L74

[wilsonianb]

Should Open Payments list endpoint query parameters then support after/before instead of just cursor?
https://github.com/interledger/rafiki/blob/3d30ce755b4218a777ec77f408be5d15dfa0e687/packages/backend/src/shared/pagination.ts#L9-L20