Add publish next change event

[taoerman]

Summary

Added a new change event for publishing next version of a channel. It is using staging tree now, we can expand to use the main tree when needed.

Verification performed:

• New unit tests ran successfully
• With storage mock in tests commented out, -next.sqlite3 file was written to Minio database

References

Issue: #4471
Previous PR: #4980

Reviewer guidance

Run unit tests
Ensure current publish logic is not affected

[rtibbles]

@taoerman sorry for the delay in review here - this all looks very straight forward.

My one thought is more about the naming than anything else - and we could potentially change that in the future. My only thought was for the event to be PUBLISHED_NEXT rather than specifying the staging tree particularly. My reason for this is that I could also see this kind of draft publishing being useful for users who are not using scripting, as currently the best way to preview a channel is by publishing it, importing to Kolibri, and then previewing there.

With this naming focus on PUBLISHED_NEXT that would just tweak the naming of the methods etc throughout. We could still just do publishing of the staging tree for now in the actual logic, and leave publishing a NEXT version of the main tree as something to happen later.

[taoerman]

@rtibbles That makes sense! I updated the name to PUBLISHED_NEXT and changed the function names too. PTAL. Thanks!

[rtibbles]

Thanks @taoerman - looking good!