Allow for marking blueprints in custom namespaces as "Hidden"

[duncanmcclean]

On most blueprints, like those for collections & taxonomies, there's an option on the edit page to mark the blueprint as "Hidden".

However, the hidden toggle isn't shown for "custom namespaces" (eg. those registered with Blueprint::addNamespace();.

In my case, with Runway, it would be nice to show this option since hiding a resource's blueprint now correlates to whether the "Create" button is displayed or not (ref: statamic-rad-pack/runway#508).

Just leaving this here as a reminder to come back to.

[mefenlon]

I have been using the hidden: true in my blueprints from #508. I have noticed that if the blueprint is editied in cp, the hidden: true line is removed.