Write enhancements

[bart-degreed]

Continuation of #851.

This PR concerns write operations and addresses several topics:

• Add POST/DELETE relationship endpoints, which enable to add/remove resources to/from a to-many relationship.
• Redesign of handling updates, where we try to fetch the minimum data required to perform the operation. If the operation fails, we run additional queries to determine why and produce a descriptive error according to json:api requirements.
• Rely more on the EF Core navigation metadata model, instead of dealing with foreign keys directly.
• Apply the whole operation in a single SaveChanges() call, removing the need for internal transaction management.
• Add resource change tracking for POST, similar to PATCH. Note that in practice, you'll only notice if using client-generated IDs.
• Refactoring of tests to hit all combinations of create/update/delete for the various endpoints and relationship kinds.
• Additional tests for corner cases like malformed request bodies, foreign key constraint violations, implicit removal in one-to-one relationships, required/optional relationships, composite foreign keys etc.

Fixes #353
Fixes #850
Fixes #852
Fixes #853
Fixes #578
Fixes #870
Fixes #790

[bart-degreed]

Replaced by #875.