pass $has_data as a param to ConfigFieldItem::settingsForm()

Coming from #2015691: Convert field type to typed data plugin for number module - #13 to #15.

We're trying to make FieldItem classes as agnostic as possible about configurable fields / base fields, and thus have them do all of their job using striclty FieldDefinitionInterface.

One issue is the settingsForm() method. Typically you need to check $field->hasData() in there and disable a couple form elements if TRUE. But hasData() is not part of FieldDefinitionInterface, and exists only for configurable fields.

No "real" problem in practice, since the fact that settingsForm() is being called means that the fields *is* a configurable field.
But strictly speaking the resulting code is a bit sloppy.

I'm not sure we need to add hasData() to FieldDefinitionInterface. Best way IMO is to let the caller (Field UI) call $field->hasData() and pass the result as an extra param to $item->settingsForm():
- An explicit param improves awareness about "i should watch this and set #access to FALSE where applicable"
- it prevents people from doing '#disabled' => $field->hasData() separately on each element (hasData() is one EFQ per call)
- that's how it worked in D7 (hook_field_settings_form), not sure why we (I) reverted that.