[API] Add Source & Destination definition management features

[thearchitector]

Managing Source and Destinations definitions programmatically seems like a really useful feature, but is only exposed via the UI; there is no documentation on how to add, update, or delete (so properly manage) custom/private connectors defined through Docker images programmatically.

I am requesting, at least, that documentation is written about how one can do this via the API; the only way to discover how to do it right now is to intercept and parse the network requests made by the UI.

As a new feature set, it would be incredibly useful to be able to check if a definition exists by docker repo, update them if possible, and delete if need be. Deletes would have to trigger cascade deletions across all Sources, Destinations, Connections, data, etc. (so that may be undesirable).