cherrypickapproved: implement org-level configuration

[xmudrii]

This PR implements the org-level configuration for the cherrypickapproved plugin. Right now, it's possible to configure this plugin only on the repo-level. If you have multiple repos, you have to repeat the config for each repo, which is inefficient and clutters the configuration file, especially if the config is the same for all repos.

With this PR, such a configuration is allowed:

cherry_pick_approved:
  - org: org-name
    branchregexp: '^release/v.*$'
    approvers:
      - ...

The repo-level config has the priority over the org level configuration. This also means that this is not a breaking change, i.e. the existing configurations will continue working as before.

/assign @saschagrunert
cc @kubernetes-sigs/release-engineering

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[xmudrii]

/test pull-prow-unit-test-race-detector-nonblocking

[matthyx]

@xmudrii you might want to take a look at #147 where I'm trying to propose a generic solution that could be applied to all plugins

[xmudrii]

In addition to #147, I'd like that we take a different route here, so I'm going to close this PR and start over again if needed.
/close

[k8s-ci-robot]

@xmudrii: Closed this PR.

In response to this:

In addition to #147, I'd like that we take a different route here, so I'm going to close this PR and start over again if needed.
/close

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