Plugin discovery and management

[rhuss]

This epic references the tasks / ideas for implementing a plugin discovery and management tooling in addition to the homebrew mechanism that we currently support.

• [ ] #1386
• [ ] https://github.com/knative/client/issues/1604
• [ ] #1231
• [x] #1368
• [ ] #1080
• [ ] #1584
• [x] https://github.com/knative/community/issues/1127

[github-actions[bot]]

This issue is stale because it has been open for 90 days with no activity. It will automatically close after 30 more days of inactivity. Reopen the issue with /reopen. Mark the issue as fresh by adding the comment /remove-lifecycle stale.

[rhuss]

/approve /lgtm