omit Groups from /Schemas if not configured

[gsar]

I noticed the /Schemas endpoint always returns Group even though we don't have groups defined in our set up. Allowing default_resources to be configurable / overridable (or even determining this automatically from the fact that there is no groups resource included via the resources mixin) is a good idea to allow auto-discovery.

[pond]

#133 will implement this, via a new call that lets you override defaults. I tried a few introspection approaches to do this automatically, but they were messy and unreliable. A simpler approach seemed better.

[pond]

Published as v2.9.0 (Rails 7) and v1.10.0 (Rails 6).