Add two new plugin events for recommendation and assign_items

[lukeIam]

Description

I wanted to be able to influence

1. recommendation behavior
2. Item to TrackInfo matching

from a plugin.
So I added those to events to the beets code.

Please let me know if you are generally interested in merging this to beets.
Then I will update the docs and changelog...

More background:
I'm using beets for sorting audiobooks using audible data. But if there is no clean match beets mixes the order of my tracks.
With that plugin event I'm able to control the matching and make sure that the order is keeps the same.

I plan to use the recommendation event to downgrade a strong recommendation if the next candidate is closer than a threshold.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[lukeIam]

example plugin: https://gist.github.com/lukeIam/d371d5f8ac53e645b6d46ece3649d1d0