Add option to hide trailing icon in DropdownMenu (#167782)

This PR adds support for hiding the trailing icon in `DropdownMenu`
widget. Currently, there's no built-in option to remove it.

The change is non-breaking, as the trailing icon remains visible by
default unless `showTrailingIcon` is explicitly set to `false`. The
`showTrailingIcon` parameter follows a similar pattern to its use in the
`ExpansionTile` widget.

Fixes #164908

## Pre-launch Checklist

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

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Co-authored-by: Qun Cheng <36861262+QuncCccccc@users.noreply.github.com>