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

aio/content/guide/testing.md

@@ -2367,9 +2367,9 @@ The [override metadata object](#metadata-override-object) is a generic defined a
 
 <code-example format="." language="javascript">
   type MetadataOverride<T> = {
-    add?: T;
-    remove?: T;
-    set?: T;
+    add?: Partial<T>;
+    remove?: Partial<T>;
+    set?: Partial<T>;
   };
 </code-example>
 
@@ -2725,9 +2725,9 @@ appropriate to the method, that is, the parameter of an `@NgModule`,
 
 <code-example format="." language="javascript">
   type MetadataOverride<T> = {
-    add?: T;
-    remove?: T;
-    set?: T;
+    add?: Partial<T>;
+    remove?: Partial<T>;
+    set?: Partial<T>;
   };
 </code-example>