Refactor backend-specific clients to use a new Backend interface

[gadomski]

Related Issue(s):

• Alternative solution to Improve separation of concerns between core and backend libraries #453 and [WIP] Generate links in api layer #472

Description:

This is a work-in-progress PR to demonstrate an alternative approach to decoupling backends from the API layer. Two main features:

• A new Backend interface that does the actual fetching from the database/data store
• A new PaginationLinks structure that allows a backend to provide "unresolved" pagination links, that are resolved at the client layer

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.

[gadomski]

Closing as OBE, but I'm going to keep the branch around as a PoC for a while.