Enable installation of CLI plugins

[triceam]

Make an action that enables the execution of cli plugin installation from a known list of plugins.

As part of this, perhaps we only show commands for installed cli extensions? (Needs more research)

Also related: have smarter feedback/help/redirect. So, when a plugin is not found, we point them to the right installation page instead of just the main cli page - which may, or may not, have any info about the specific missing plugin.