Distribute kubectl-retina in krew

[rbtr]

kubectl-retina is currently distributed as a binary in the release artifacts. It would significantly improve the UX to distribute it via Krew.

### Tasks
- [ ] https://github.com/microsoft/retina/pull/126
- [ ] https://github.com/microsoft/retina/pull/193
- [ ] https://github.com/microsoft/retina/pull/194

[rbtr]

for most complete UX this probably requires that we build kubectl-retina for all supported platforms of kubectl

[rbtr]

depends on #100

[rbtr]

once the plugin is accepted we should follow their recommendation and automate releases

https://krew.sigs.k8s.io/docs/developer-guide/release/automating-updates/ https://github.com/rajatjindal/krew-release-bot

[nddq]

All related PRs have been merged 🚀