Description

This change introduces a BottomSheetTheme that allows you to theme color, elevation, and shape of BottomSheet. This can be done at the theme level and at call time of showBottomSheet and showModalBottomSheet. See the following image for an example (with some questionable design choices):

Related Issues

Closes #26854
Closes #30444

Tests

I added the following tests:

• Tests for the theme and modifying the color/elevation/shape of the BottomSheet itself
• Tests for ensuring the showBottomSheet and showModalBottomSheet pass through color/elevation/shape to the BottomSheet

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (Please read Handling breaking changes). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.