Expose all the relevant attributes of TextFormField.

[Abhilash-Chandran]

In its current, many of TextformField attributes are hidden from the user. While not all of attribute are relevant, a number of attributes should be exposed to the user. Following are relevant fields I find missing.

1. FocusNode focusNode
2. TextInputAction textInputAction
3. StrutStyle strutStyle
4. TextDirection textDirection
5. TextAlign textAlign
6. TextAlignVertical textAlignVertical
7. bool autofocus
8. bool readOnly
9. ToolbarOptions toolbarOptions
10. bool showCursor
11. ValueChanged<String> onChanged already done
12. GestureTapCallback onTap
13. VoidCallback onEditingComplete
14. ValueChanged<String> onFieldSubmitted already done
15. FormFieldSetter<String> onSaved

If someone else finds any other relevant field missing, please comment below those fields.