adds an exposed sheet contorller to the better feedback controller

[caseycrogers]

📜 Description

This PR exposes a DraggableScrollableController from FeedbackController.
This was a PR I intended to do a long time ago, but it was blocking on a PR landing in flutter/flutter and that took so long that I forgot to get back to this ):

I have a handful of TODOs in other places that this PR unblocks, but I figured I'd do those in a follow up PR as they're much fussier and harder to land.

💡 Motivation and Context

This PR allows users of BetterFeedback to listen to the state of the feedback sheet and manually manipulate it. This is useful for things like:

1. Manipulating the sheet from within your custom sheet content
2. Driving animations from within your sheet

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

1. Remove the public drag handle widget and automatically embed it in the user's sheet gated behind a bool in the feedback theme
2. Replace sheetProgress in the places where it is used with a direct reference to the sheet controller
   (blocking on a PR to add double get progress to flutter/flutter Add progress, sizeToprogress and progressToSize to DraggableScrollableSheet flutter/flutter#135366)

[ueman]

Awesome thanks!