cli manager optional operator must have a git server that supports clone and fetch operations executed by Krew https://github.com/kubernetes-sigs/krew/blob/efaaa22897a069937e3017dbeb01dfeb6a6aec80/internal/gitutil/git.go#L30-L63 

Currently, cli manager code base only responses git clone request (partially) https://github.com/openshift/cli-manager/blob/ff674b67fa3e979ce85793c591636dd53ea98445/pkg/server/v1/plugins.go#L274-L293 and does not support git fetch.

Acceptance criteria;

• support git clone
• support git fetch
• propose a way of authentication mechanism