Add the possibility to specify webhook annotations

[fllaca]

This is needed to tweak the integration with CertManager: https://cert-manager.io/docs/concepts/ca-injector/

I opened this for release-0.8 because we use stash 0.8.3, but I guess it can be easy to replicate in latest master if you are happy with this PR

[fllaca]

@tamalsaha not sure if this is the proper way of proposing changes to this repo, please let me know if I need to go other way.

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