Add manually triggered mode for zero plugin

[SJoshBrown]

I'm fairly new to the process so I hope I've done this correctly. Please let me know if I need to make any changes to get this merged up.

This is for Issue #2274

[sampsyo]

This looks fantastic! Thank you for adding tests and amending the docs too.

Since it looks like some code was duplicated between the new process_item method and the old write_event, could you please factor out the common code so we only have one copy?

[SJoshBrown]

I've taken another pass at this and factored out some of the shared code. I also added a few more tests to increase the coverage.

#2274

[nathdwek]

I'm interested in this feature being added for consistency's sake, so I took a look at the whole plugin.
There is only one important thing I think should be added before going through with this, that is adding safeguards in manual mode when the query is empty, because of the nature of the operation (see #1286 )

However, I think the original plugin is a mess in terms of style and structure, cf my review. Maybe this is the occasion to revisit that?

E: Ok so it turns out you cannot review untouched code. I'll do a separate PR for that.

[sampsyo]

Indeed, the plugin as it was was a bit of a mess. Thanks for looking into it, @nathdwek!

[nathdwek]

I opened #2345 . Some unexpected issues in the testing department, but would you be able to rewrite on top of that when that's fixed? We can also do it the other way around if it's more convenient for you?

[SJoshBrown]

I'm happy to wait until it gets some cleaning up. Then I can come back and rewrite this

[nathdwek]

I'll see tomorrow if I can sort this test thing out with a fresh mind. But my absolute priority is not getting in your way!

[sampsyo]

Either way seems to work from my perspective—I'd also be happy to merge this new feature first and then do the refactoring afterward.

[SJoshBrown]

@sampsyo @nathdwek, I think this should be ready to go now. Let me know if there are any other changes we should make

[sampsyo]

All merged up. ✨ Thank you for taking care of this! It looks perfect.