Move cherry picking functionality over to Prow

[jennybuckley]

#7332

Moves experiment/cherrypicker under prow/external-plugins as suggested in #7332 (comment)

Is this really enough to get it running in k/k though?
I think it also needs a deployment and service, etc, like external-plugins/needs-rebase has.

[BenTheElder]

I think it also needs a deployment and service, etc, like external-plugins/needs-rebase has.

It'll need those, we'll want to fix up the build to look like the needs-rebase and add it to the deployment bundle. I think those can be other PR(s) though.

/cc @Kargakis

[BenTheElder]

needs a hack/update-bazel.sh to update the build.

[0xmichalis]

Please, leave a breadcrumb in experimenta;/cherrypicker that points to the new location.

[jennybuckley]

@Kargakis Is this okay for the breadcrumb?

[0xmichalis]

yeah, thanks

[0xmichalis]

/lgtm

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