Bottom appbar/sheet shadow property

[esouthren]

Revised re-land of https://github.com/flutter/flutter/pull/119819/. Fixes https://github.com/flutter/flutter/issues/118150 and https://github.com/flutter/flutter/issues/118244.

Instead of hard-defaulting to no shadow in M3, we add a shadowColor property to bottomAppBar and bottomSheet, and their respective themes.

The M3 default is then set to Colors.transparent shadow, but can be overridden by users.

Updated M3 defaults:

Internal testing has been conducted and g3fix created (cl/512056372).

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] All existing and new tests are passing.