Add endpoint for contextual catalog

[kahboom]

Is your feature request related to a problem? Please describe.
Right now, the catalog endpoint returns all possible steps. If you are trying to add a new step onto an existing step, there is no way to know which steps are possible to connect to it.

Describe the solution you'd like
I would like to be able to call an endpoint that accepts context, maybe via a query param, to return steps that can be connected to the current step.

Describe alternatives you've considered
Creating a custom validation on the frontend, which I will be doing anyway, as a way to filter the existing steps. This wouldn't scale well though.

Additional context
This feature would be to support the Mini Catalog in the frontend (KaotoIO/kaoto-ui#216), which is not available yet.