Generate mocks with make gogen

[WillAbides]

I'm not sure this should be an enhancement, but it seems like the best fit of the issue templates.

Summary

Generate mocks with make gogen.

Motivation

I was doing some recreational coding to explore argo-cd and made a change to a mocked interface. This caused build errors that I expected make gogen to fix. I did some searching and eventually found a PR comment by @alexmt with instructions. The comment also says "We definitely should automate mocks generation", so I decided to have a go at that.

Proposal

Add //go:generate comments that generate mocks. I've coded this up and will make a companion PR for this.