Add possibility to change credentials

[Bert-R]

This is an enhancement request, to make the highly appreciated FordPass integration even better.

It would be useful to extend the the app with a configuration screen to update the login credentials (username and password). The request came up in #435 (comment) and #442 (comment) and I for my self would also like to use it, to set a dedicated account for the integration. Besides this, it is good as a general security improvement: people now stick with existing credentials because of the burden of change, even though they have reasons to changes the credentials.

In the interim, it would be good to document the manual procedure to "edit the HA database and change the saved values for Fordpass username and password", perhaps as a comment on this ticket.