93803: Stricter typing for dataService decorator & LinkService

The initial idea was to type dataService decorator strictly to BaseDataService.
However, HrefOnlyDataService should not expose methods other than findByHref & findAllByHref, but must still work with LinkService.

To address this we introduce HALDataService: an interface with the minimal requirements for a data service to work with HAL links
- dataService decorator can only decorate a class that implements HALDataService
- services retrieved from DATA_SERVICE_FACTORY should therefore work in LinkService

src/app/core/data/base/hal-data-service.interface.ts

@@ -0,0 +1,41 @@
+/**
+ * The contents of this file are subject to the license and copyright
+ * detailed in the LICENSE and NOTICE files at the root of the source
+ * tree and available online at
+ *
+ * http://www.dspace.org/license/
+ */
+import { Observable } from 'rxjs';
+import { FollowLinkConfig } from '../../../shared/utils/follow-link-config.model';
+import { RemoteData } from '../remote-data';
+import { FindListOptions } from '../find-list-options.model';
+import { PaginatedList } from '../paginated-list.model';
+import { HALResource } from '../../shared/hal-resource.model';
+
+/**
+ * An interface defining the minimum functionality needed for a data service to resolve HAL resources.
+ */
+export interface HALDataService<T extends HALResource> {
+  /**
+   * Returns an Observable of {@link RemoteData} of an object, based on an href,
+   * with a list of {@link FollowLinkConfig}, to automatically resolve {@link HALLink}s of the object
+   *
+   * @param href$                       The url of object we want to retrieve. Can be a string or an Observable<string>
+   * @param useCachedVersionIfAvailable If this is true, the request will only be sent if there's no valid cached version.
+   * @param reRequestOnStale            Whether or not the request should automatically be re-requested after the response becomes stale
+   * @param linksToFollow               List of {@link FollowLinkConfig} that indicate which {@link HALLink}s should be automatically resolved
+   */
+  findByHref(href$: string | Observable<string>, useCachedVersionIfAvailable?: boolean, reRequestOnStale?: boolean, ...linksToFollow: FollowLinkConfig<T>[]): Observable<RemoteData<T>>;
+
+  /**
+   * Returns an Observable of a {@link RemoteData} of a {@link PaginatedList} of objects, based on an href,
+   * with a list of {@link FollowLinkConfig}, to automatically resolve {@link HALLink}s of the object
+   *
+   * @param href$                       The url of list we want to retrieve. Can be a string or an Observable<string>
+   * @param findListOptions             The options for to use for this find list request.
+   * @param useCachedVersionIfAvailable If this is true, the request will only be sent if there's no valid cached version.
+   * @param reRequestOnStale            Whether or not the request should automatically be re-requested after the response becomes stale
+   * @param linksToFollow               List of {@link FollowLinkConfig} that indicate which {@link HALLink}s should be automatically resolved
+   */
+  findAllByHref(href$: string | Observable<string>, findListOptions?: FindListOptions, useCachedVersionIfAvailable?: boolean, reRequestOnStale?: boolean, ...linksToFollow: FollowLinkConfig<T>[]): Observable<RemoteData<PaginatedList<T>>>;
+}