downstream `opm alpha diff` moving to `oc-mirror`

 – NOT A BUG –
This was a story, but CI is not working for OLM project, so moved to OCPBUGS where it is. 

----------------------------

upstream the `opm alpha diff` functionality moved to `oc-mirror` team by a non-RH actor.

This story is to track downstreaming the two PRs.

The only thing to verify here is that there is no more `opm alpha diff` command. 

Other changes in the PRs are to externalize some interfaces and implement an undocumented alpha-level internal channel-level property list.