Allow control on DraggableSheet

[apomalyn]

Is your feature request related to a problem? Please describe.

As a developer, I would like to have control of the DraggableSheet used when sheetIsDraggable is true.
In my specific case, I have a form with multiple fields on it, so I want to expand the sheet to fill the screen when the user focus on the form.

Describe the solution you'd like

Pass the DraggableSheetController or BetterFeedback.of(context).expandSheet().

Describe alternatives you've considered

Something similar was proposed in the PR #88

[ueman]

Should be closed via #239