Re-implement `assign` plugin from prow

[djzager]

https://prow.k8s.io/plugins

Description
The assign plugin assigns or requests reviews from users. Specific users can be assigned with the command '/assign @user1' or have reviews requested of them with the command '/cc @user1'. If no users are specified, the commands default to targeting the user who created the command. Assignments and requested reviews can be removed in the same way that they are added by prefixing the commands with 'un'.
Events handled
[GenericCommentEvent (any event for user text)]

https://github.com/kubernetes/test-infra/tree/master/prow/plugins/assign

[djzager]

/kind feature
/triage accepted
/priority important-soon

[konveyor-ci-bot]

This issue is currently awaiting triage.
If contributors determine this is a relevant issue, they will accept it by applying the triage/accepted label and provide further guidance.
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[djzager]

/kind feature
/triage accepted
/priority important-soon