* feat: Add subscription CRUD
- Add kn subscription command group and CRUDL sub-commands
- create/update uses following flag names for better alignment:
- --sink for subscriber
- --sink-reply for reply
- --sink-dead-letter for dead-letter-sink
- Add 'subscriptions' and 'sub' aliases
- Introduce shared library `knative.dev/client/lib/printing`
to print Sink object in describe output
* Set default channel type messaging.knative.dev/v1beta1:Channel
i.e. if no prefix is given to `--channel`, consider it of `Channel` type
* Update e2e tests
* Update channel flag description
Navid Shaikh