Add --plaintext flag for pushing to http registries such as localhost

[blairdrummond]

Hello! I was trying out the policy command with a localhost registry for testing, and discovered that I couldn't push to a localhost registry on http. The insecure flag skips TLS Verification, but using https as the Scheme in the docker registry was hard-coded. This PR adds a --plaintext flag which allows this to be configurable.

I did a quick comparison with other tools to come up with the name. The lingo of --plaintext matches ArgoCD's behaviour. In crane they just use --insecure to mean "connect to the registry over http".

Comparison with other tools

ArgoCD

--insecure = no TLS verify
--plaintext = Use HTTP client/server

• Link to doc

Crane's --insecure option allows for HTTP scheme on registry

• doc

• how it's used