Add specification for sources #79
Description
The source specification hasn't been moved to the repository (yet).
At the minimum the specification should include the source duck type (in the document linked above).
I believe specific sources, such as PingSource, ApiServerSource, are out-of-scope, as I don't except multiple implementations of those (except single-tenant vs multi-tenant variants). If someone think otherwise, please add a comment below.
From a conformance PoV, I wonder if we should introduce a new Source CRD, similar to Channel, delegating the actual behavior to underlying implementations. Obviously the notion of default source does not exist. Anyway, something to think about.