Dynamic entry type options fields

[vassalloandrea]

Until now we can update the option_type adding the options using a custom JSON fields. The problem is that we need to save different fields, which is mandatory, for each provider.

With this PR, we added the ability to render the correct option form based on the chosen provider during the option type creation. This means that the admin user can't change the provider of an option type if it has already been created.

[elia]

BTW thanks for getting after and solving this problem! As already stated elsewhere this is really great stuff! 👏