Add support for providing sink spec in GVR format

[vyasgun]

Feature request

The user can specify a sink in subscription create and update (--sink, --sink-reply, --sink-dead-letter flags), source create and update(--sink flag), and broker create and update (--dl-sink flag). Currently, only a few known types are supported as an argument to these flags ("broker", "ksvc", and "channel"). If the user provides any other prefix outside the supported ones, kn returns an unsupported sink prefix error.

Use case

To use sinks of custom types.

UI Example

kn source ping create my-ping --schedule "*/2 * * * *" --data '{ value: "hello" }' --sink <apiVersion>:<kind>:<name>:<namespace>