Add support for `TextFormField` in `getText` command

[tnmma96]

I'm running into an issue in which command getText returns an empty string from a TextFormField widget.
I notice that ElementHelper.getText() method hasn't supported retrieving text from this widget class, can you take a look and add TextFormField retrieval logic to this method?
Thank you in advance.

appium-flutter-server/server/lib/src/utils/element_helper.dart

Lines 171 to 185 in 78ba97a

	if (widget is Text) {
	if (widget.data != null) {
	buffer.write(widget.data);
	} else if (widget.textSpan != null) {
	buffer.write(widget.textSpan!.toPlainText());
	}
	} else if (widget is RichText) {
	buffer.write(widget.text.toPlainText());
	} else if (widget is EditableText) {
	buffer.write(widget.controller.text);
	} else if (widget is TextField) {
	buffer.write(widget.controller?.value.text);
	} else if (widget is ButtonStyleButton) {
	buffer.write(getElementTextRecursively(widget.child, visited: visited));
	}