This PR allows developers to add listeners on DraggableScrollableController that fire whenever an attached sheet's size changes. Listening for changes is already possible via DraggableScrollableNotification. This PR is still useful because:

1. Depending on the situation, it is often much easier to register a listener on a controller than by using a notification listener. See: https://github.com/ueman/feedback/blob/642bcea820708cf0490a31209c928c03e04c6b2d/feedback/lib/src/feedback_widget.dart#L61
2. Other controllers (such as scroll controller) are change notifiers so developers will expect the same form DraggableScrollableController.

Fixes #96088

Pre-launch Checklist

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

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