Make WorkflowsFieldItem implement OptionsProviderInterface

Yes it will.

Could at the same time get rid of the custom formatter and replace it with 'list_default' but Drupal\options\Plugin\Field\FieldFormatter\OptionsDefaultFormatter hardcoded the property name to 'value', so would have to change the property name from the current 'state'.

Yes, it gets the labels via OptionsProviderInterface::getPossibleOptions() so it turns out just fine.

Here's an initial patch, mostly based on state_machine, so all credit to @bojanz.

Notes:
1. to preserve relative ordering of states used WorkflowTypeInterface::getStates() and filtered it with StateInterface::canTransitionTo(), instead of using WorkflowTypeInterface::getTransitionsForState() approach
2. WorkflowsFieldItem::getWorkflow() gets called from WorkflowsFieldItem::applyDefaultValue() quite early (at FieldItemInterface::storageSettingsForm() stage) so changed its implementation to return NULL when it is unknown, will need to look more into it to make sure ::applyDefaultValue() does what I think it does ;)
3. keeping current state needs explicit 'keep at' transitions (as is the case in core now)

Should work, but still needs work, will get back to it in a week or so, NR just for testing.

This should do it.