Improve type selector on flows form

1. Users do not need to know about the "type" of flow and the difference between a normal flow and a third-party flow. As suggested here, we can consider using something like "custom configuration".

   

2. We should update the glossary to avoid the Third-party flow term.

Note that UI in 1. is currently only shown when both feature flags ai_catalog_flows and ai_catalog_third_party_flows are enabled.