Implement `$floor` arithmetic aggregation operator

[b1ron]

What should be done?

Refs #1453.

• https://www.mongodb.com/docs/manual/reference/operator/aggregation/floor/

[Artyom Fadeyev]

@AlekSi Hey, can I try this?
Or are there any more suitable and relevant candidates?

[Alexey Palazhchenko]

@fadyat We are in the process of transitioning to 2.0 right now: https://blog.ferretdb.io/new-ferretdb-v121-release/#ferretdb-v20-its-coming Doing that there would be a step sideways.

I will try to update good first issue Good issues for new external contributors soon, please check it in a couple of days