Take review preferences into account when determining reviewers

[Kobzol]

This PR implements step 6 of #1753. When [assign.review_prefs] is configured in triagebot.toml, triagebot will now take into account the number of PRs already assigned to a reviewer, and their reviewer preferences.

Users can set their review preferences by sending a DM to triagebot on Zulip, and sending work set-pr-limit <max-prs> (this was implemented in #1919). When a reviewer already has N PRs assigned where N >= max-prs, they will not be considered for assignment.

Note that the way this is currently implemented, the assigned PR tracking only works for rust-lang/rust, so it should not be enabled on any other repo.

There are a couple of questions in the design that we should consider (they do not necessarily block this PR though):

• Should the assigned PR tracking be within a repository or across repositories? In other words, do reviewers want to configure a "max assigned PR limit" across the whole of rust-lang, or do they want to have a limit per repository?
• Should we only consider open PRs or open + draft PRs to be assigned to a given reviewer?
• Should we ignore this limit in certain situations, e.g. when you request a reviewer explicitly (r? reviewer) or when we use the adhoc group fallback assigment logic?

CC @apiraino

Requesting a review regarding the implementation details, but I still want to discuss the design on Zulip first and update Forge, so marking as a draft for now. Best reviewer commit-by-commit. I should probably add more logging before we merge this.