Introduce InclusionResolver concept

[bajtos]

Introduce the concept of a inclusion resolver, which is a function we can use to fetch related models (both in our ORM/REST API but also from GraphQL resolvers in the future).

/**
 * @returns An array of resolved values, the items must be ordered in the same
 * way as `sourceEntities`. The resolved value can be one of:
 * - `undefined` when no target model(s) were found
 * - `Entity` for relations targeting a single model
 * - `Entity[]` for relations targeting multiple models
 */
export type InclusionResolver = (
  /**
   * List of source models as returned by the first database query.
   */
  sourceEntities: Entity[],
  /**
   * Inclusion requested by the user (e.g. scope constraints to apply).
   */
  inclusion: Inclusion,
  /**
   * Generic options object, e.g. carrying the Transaction object.
   */
  options?: Options,
) => Promise<(Entity | undefined)[] | (Entity[] | undefined)[]>;

See #3387 for more details & a prototype implementation.

Acceptance criteria

• Introduce a new type InclusionResolver
• Implement a new helper function includeRelatedModels, it will become a part of a public API of @loopback/repository. Under the hood, this function should leverage inclusion resolvers to fetch related models.
• Write unit-level tests to verify the implementation, use mocked or stubbed inclusion resolvers.

Note: helper name includeRelatedModels is not final, feel free to propose a better one!