Add findByForeignKeys helper (initial version)

[bajtos]

Implement a new helper function findByForeignKeys, it will become a part of a public API of @loopback/repository.

Signature:

function findByForeignKeys<
  Target extends Entity,
  TargetID,
  TargetRelations extends object,
  ForeignKey
>(
  targetRepository: EntityCrudRepository<Target, TargetID, TargetRelations>,
  fkName: StringKeyOf<Target>,
  fkValues: ForeignKey[],
  scope?: Filter<Target>,
  options?: Options,
): Promise<(Target & TargetRelations)[]>;

The initial version should be simple, "inq splitting" and additional "scope" constraints are out of scope of this task.

Example implementation - I think it pretty much covers the full scope of this task, but obviously we need to add test coverage as explained in the acceptance criteria. It would be also great to throw an error if scope argument was provided, because we don't support scopes yet.

const where = ({
  [fkName]: fks.length === 1 ? fkValues[0] : {inq: fkValues},
} as unknown) as Where<Target>;
const targetFilter = {where};

return targetRepository.find(targetFilter, options));

See #3387 for more details & a prototype implementation.

Acceptance criteria

Beside the implementation, it's important to create testing infrastructure that will make it easy to add new tests in the future. There should be two kinds of tests:

• Unit-level tests inside packages/repository, these tests should mock targetRepository. By using a mock repository, we can assert on how many queries are called, inject errors to verify how are they handled, etc.
• Integration-level tests inside packages/repository-tests, these tests will call real repository class & database, we will invoke them against different connectors (MySQL, MongoDB, and so on).

Out of scope

• Support inq splitting in findByForeignKeys Support inq splitting in findByForeignKeys #3444
• Include related models with a custom scope Include related models with a custom scope #3453