Add AppBar.forceMaterialTransparency (#101248)

[monkeyswarm]

Allows gestures to reach widgets beneath the AppBar (when Scaffold.extendBodyBehindAppBar is true).

Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.

Fixes https://github.com/flutter/flutter/issues/101248 Previous discussion at previous strategy https://github.com/flutter/flutter/pull/102055 suggested using a new flag on AppBar, which this PR does, instead of attempting to derive intended MaterialType from color/elevation. Naming convention open for discussion.

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 NOTE there are Pixel tests failing at head, unrelated to this PR.

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