Add extension copy command

Created by: chrismwendt

A customer is having difficulty publishing language extensions to their private registry due to internet restrictions, OS package manager restrictions, and an incompatible Node version.

This PR adds the following command:

src extension copy -extension-id=sourcegraph/java -current-user=chris

Internally, it fetches the manifest and bundle from Sourcegraph.com (hard-coded) then publishes them to the current Sourcegraph instance.