Allow users to specify which plugin to install in "Install Plugin from Source" command

[JasonYeMSFT]

The current implementation as of 1.132.0 let user enter a owner/repo, or git url or local path to install a plugin from source. This is not enough to determine which plugin to install when a remote git repo has more than one plugins in it. It is natural to host multiple plugins within one repository since the marketplace manifest spec supports declaring multiple plugins. Both Copilot CLI and Claude Code supports specifying which plugin to install in the installation command using the plugin@marketplace argument.

[connor4312]

This works for me. Make sure your repo is a marketplace.