refactor(table): allow readonly array in data source input (#13137)

Adds `ReadonlyArray` and `Observable<ReadonlyArray>` to the list of accepted types for the `CdkTable.dataSource` input.

Fixes #13126.

src/cdk/table/table.ts

@@ -64,6 +64,13 @@ export interface RowOutlet {
   viewContainer: ViewContainerRef;
 }
 
+/**
+ * Union of the types that can be set as the data source for a `CdkTable`.
+ * @docs-private
+ */
+type CdkTableDataSourceInput<T> = DataSource<T> | Observable<ReadonlyArray<T> | T[]> |
+                                  ReadonlyArray<T> | T[];
+
 /**
  * Provides a handle for the table to grab the view container's ng-container to insert data rows.
  * @docs-private
@@ -310,13 +317,13 @@ export class CdkTable<T> implements AfterContentChecked, CollectionViewer, OnDes
    * subscriptions registered during the connect process).
    */
   @Input()
-  get dataSource(): DataSource<T> | Observable<T[]> | T[] { return this._dataSource; }
-  set dataSource(dataSource: DataSource<T> | Observable<T[]> | T[]) {
+  get dataSource(): CdkTableDataSourceInput<T> { return this._dataSource; }
+  set dataSource(dataSource: CdkTableDataSourceInput<T>) {
     if (this._dataSource !== dataSource) {
       this._switchDataSource(dataSource);
     }
   }
-  private _dataSource: DataSource<T> | Observable<T[]> | T[] | T[];
+  private _dataSource: CdkTableDataSourceInput<T>;
 
   /**
    * Whether to allow multiple rows per data object by evaluating which rows evaluate their 'when'
@@ -759,7 +766,7 @@ export class CdkTable<T> implements AfterContentChecked, CollectionViewer, OnDes
    * render change subscription if one exists. If the data source is null, interpret this by
    * clearing the row outlet. Otherwise start listening for new data.
    */
-  private _switchDataSource(dataSource: DataSource<T> | Observable<T[]> | T[]) {
+  private _switchDataSource(dataSource: CdkTableDataSourceInput<T>) {
     this._data = [];
 
     if (this.dataSource instanceof DataSource) {