Sample source plugin

[rhuss]

A sample source plugin should be created to showcase how a source plugin could be bootstrapped.
It could piggyback on an existing source.

I'd suggest to move this source plugin as a whole project into a subdirectory with an own go.mod, and in the future we might want to add other 'specializations' of kn plugins for samples.

Origin: knative/client#520

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[rhuss]

/remove-lifecycle stale

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.