Support for default timezone value.

Here a patch that provides this option in the field config page:

The last submitted patch, 2: 3359231-default-timezone-support-2.patch, failed testing. View results →

Here a new patch that shouldn't break the tests.

Here is an updated patch which should work with Drupal 11

Here is an additional change to this patch to allow us to select a default timezone without selecting a default date.

It will also set the default value of the timezone to the users (system) timezone when no default timezone is configured in the field settings.

I like the proposal and the patch! Can we get a merge request with it please? :)

Also some basic test coverage of the code would be much apprechiated!