[FEAT] Endpoint for fetching Schemas that have Custom Endpoints

[KostasFeg]

Is your feature request related to a problem? Please describe.
As another filtering option on the Custom Endpoints tab, we would like to show endpoints related to certain Schemas in order to present them as a drop-down menu.

Describe the solution you'd like
A new endpoint filtering the endpoints that have Custom Endpoints.

Describe alternatives you've considered
This could be achieved on the front-end side but would not be as clear and concise.

Additional context
Add any other context or screenshots about the feature request here.