Improve data fetching by using new backend endpoints

[Callum0x50]

This PR:

• Improves parts of the codebase where we used to get a list of resources and loop through them to find one single item. This was necessary because we had no way to query specific objects in the backend. With the recent additions to the backend, we can now refactor this code.
• Updates OAuth client configuration - Now uses the shared Developer Platform apps for local development instead of the old Container Platform CLI. This is only relevant for local dev as production has injected pipeline values.

[Callum0x50]

Depends on a backend release. Merge as soon as this release is live:
https://github.com/intility/developer-platform-backend/pull/694