refactor(core): use Partial<T> for MetadataOverride (#24103)

Allows to write:

const fixture = TestBed
      .overridePipe(DisplayNamePipe, { set: { pure: false } })
      .createComponent(MenuComponent);

when you only want to set the `pure` metadata,
instead of currently:

const fixture = TestBed
      .overridePipe(DisplayNamePipe, { set: { name: 'displayName', pure: false } })
      .createComponent(MenuComponent);

which forces you to redefine the name of the pipe even if it is useless.

Fixes #24102

PR Close #24103

tools/public_api_guard/core/testing.d.ts

@@ -55,9 +55,9 @@ export declare class InjectSetupWrapper {
 
 /** @experimental */
 export declare type MetadataOverride<T> = {
-    add?: T;
-    remove?: T;
-    set?: T;
+    add?: Partial<T>;
+    remove?: Partial<T>;
+    set?: Partial<T>;
 };
 
 /** @experimental */