This PR is to create a SearchAnchor widget in which we can use an anchor to trigger a search view. Typically an anchor is a SearchBar, or IconButton.

In order to use the SearchAnchor with the new Material 3 colors, turn on the useMaterial3 flag in the ThemeData:

  return MaterialApp(
    theme: ThemeData(useMaterial3: true),
    // ...
  );

Fixes #117483.

This is an example to show how we can use the SearchAnchor.bar() constructor. The sample code is in examples/api/lib/material/search_anchor/search_anchor.0.dart

Same example running on mobile devices shows full screen.

Another example in examples/api/lib/material/search_anchor/search_anchor.1.dart is to show how we can use SliverAppBar and SearchAnchor to get a pinned/floating effect. To achieve this, we need to add an additional api clipBehavior to AppBar, otherwise the shadow might be clipped.
See the commit.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.