feature(source binding): Support for Sink Binding as source

[rhuss]

Sink bindings are managed like any other source. Sinks are specified as usual (with prefix and name),
'subjects' (the other end of the binding) is managed via a shortcut notation:

• with name: <kind>:<apiVersion>:<name>
• with label selector: <kind>:<apiVersion>:key1=value1,key2=value2

With --subject-namespace and additional namespace can be provided (shoudl be possible for a sink, too but is not yet) As discussed on Slack, neither for a sink nor a subject the namespace is setable by the user. So I removed that option.

The implementation already uses the new sink binding from the sources.knative.dev group
and hence is a bit inconsistent to the still old usage kf sources.eventing.knative.dev for apiserver source and cronjob.

However as we will move over to sources.knative.dev very soon (right after v0.12.0) release,
this is was more appropriates.

Still WIP, but eventually fixes #624

Task list:

• create
• update
• delete
• describe
• list

[rhuss]

One question to the community: I would like to include this feature into 0.12.0 which is due tomorrow. However, because of time constraints and outstanding review, I would suggest to postpone the release to Wednesday (and I would some help with some quick reviewers ;-)

wdyt ? We can discuss this tomorrown on the WG call, however as I'm on a train at that time, not sure how it will work out for me.

// cc: @sixolet @maximilien @navidshaikh

[rhuss]

ready for review

[maximilien]

Did not manually test. Left various small comments. LGTM

[knative-prow-robot]

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This pull-request has been approved by: maximilien, rhuss

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[rhuss]

@navidshaikh @maximilien just worked on the comments, thanks for the feedback ! Very helpful.

Going to push in a second with most of the comments fixed. Some are discussable but I'm sure we can resolved those, too.

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-client-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/kn/commands/flags/sink.go	78.3%	79.2%	0.9
pkg/kn/commands/source/binding/binding.go	Do not exist	58.8%
pkg/kn/commands/source/binding/create.go	Do not exist	81.6%
pkg/kn/commands/source/binding/delete.go	Do not exist	85.7%
pkg/kn/commands/source/binding/describe.go	Do not exist	90.8%
pkg/kn/commands/source/binding/flags.go	Do not exist	91.3%
pkg/kn/commands/source/binding/list.go	Do not exist	78.3%
pkg/kn/commands/source/binding/update.go	Do not exist	74.4%
pkg/sources/v1alpha1/binding_client.go	Do not exist	85.9%
pkg/sources/v1alpha1/binding_client_mock.go	Do not exist	90.9%
pkg/sources/v1alpha1/client.go	Do not exist	100.0%

[navidshaikh]

/lgtm