Add support for mergeable rules

[AleksandarSavchev]

How to categorize this PR?

/kind enhancement

What this PR does / why we need it:

Which issue(s) this PR fixes:
Part of #687

Special notes for your reviewer:

Release note:

NONE

[gardener-prow]

Skipping CI for Draft Pull Request.
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[gardener-prow]

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[gardener-prow]

PR needs rebase.

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