improve plugin fetching code for containers and app

[illume]

Some improvements for plugin fetching code. Used to build the container images and apps.

Acceptance criteria

• Code for fetching app, and container manifest plugins should be unified and put inside headlamp-plugin
  • There should be an end to end test
  • There should be documentation for how to use this
• app fetching of plugins should compare to checksum
• container fetching of plugins should compare to checksum
• changing the fetch plugins code, or container manifest should trigger container builds on CI
• changing the fetch plugins code, or app manifest should trigger app builds on CI

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/lifecycle stale