Feat: Add yearShape property to DatePickerThemeData (#163909)

Feat: Add yearShape property to DatePickerThemeData
fixes: #163340 

## 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.

---------

Co-authored-by: Tong Mu <dkwingsmt@users.noreply.github.com>

dev/tools/gen_defaults/lib/date_picker_template.dart

@@ -50,6 +50,7 @@ class _${blockName}DefaultsM3 extends DatePickerThemeData {
         // TODO(tahatesser): Update this to use token when gen_defaults
         // supports `CircleBorder` for fully rounded corners.
         dayShape: const WidgetStatePropertyAll<OutlinedBorder>(CircleBorder()),
+        yearShape: const WidgetStatePropertyAll<OutlinedBorder>(StadiumBorder()),
         rangePickerElevation: ${elevation("md.comp.date-picker.modal.range-selection.container")},
         rangePickerShape: ${shape("md.comp.date-picker.modal.range-selection.container")},
       );