Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Reorganizing more webhook code

ref: kubernetes/features#492

Continue on https://github.com/kubernetes/kubernetes/pull/55132.

With this PR, all code shared between the mutating and validating webhook plugins is extracted into its own package.