Include default working widget

This is basically an API module at this point. There is no actual field widget usable on an entity form out of the box, just an abstract class and some suggestions for how to implement. We should provide the most basic widget that allows for adding a schema and UI schema directly in the field settings form, in a textarea. This will allow people to try out the module, get a sense of how it works, even if they ultimately plan to implement their own custom widget.

It would also let us easily QA new MRs via tugboat previews, which are already building.

It minimum, it should validate the JSON and UI schemas on submit. Bonus points for a more friendly JSON editor in the config, but maybe not worth the effort. Could support jsoneditor if present, but that could be a follow-up issue.

Most of this my already be implemented in #3565788: Widgets, views support, including changes from #3565343 and #3565448. If so let's just create an MR specific to that widget.

I am on the fence on whether this should block a 1.0 release. It would certainly be ideal but I also think we need a stable release of this module before we can make this dependency for its old parent module DKAN, so we may just go ahead with an "API-only" 1.0 release if this takes too long.