Refactor volvooncall to use ConfigFlow (#76680)

* refactor volvooncall to use ConfigFlow

* remove unused constant SIGNAL_STATE_UPDATED

* implemented feedback

* improve ConfigFlow UX by giving an option for region=None

* implemented more feedback

* next round of feedback

* implemented more feedback

* improve test coverage

* more test coverage

* Apply suggestions from code review

* implemented feedback on tests

* Apply suggestions from code review

Co-authored-by: Martin Hjelmare <marhje52@gmail.com>