Add command for finding inactive users and moving users to alumni

[Kobzol]

This PR adds two new commands. The first one helps finding users who haven't sent a message on the Rust Zulip for a long time and/or haven't sent a comment on the rust-lang GitHub organization for a long time.

The second command moves a given user to alumni in the teams that they are a member of (optionally filtered to a subset of the teams).

The motivation for this is https://github.com/rust-lang/leadership-council/blob/main/policies/membership/auto-alumni.md - we want to have some automation to help LC detect people who have not been active in the Project for some time, and then move them to alumni if they don't respond.

[marcoieni]

ci is failing ❌

[github-actions]

Dry-run check results

[WARN  rust_team::sync] sync-team is running in dry mode, no changes will be applied.
[INFO  rust_team::sync] synchronizing crates-io
[INFO  rust_team::sync] synchronizing github

[Kobzol]

@jieyouxu Could you please create an alumni label? I'll modify the PR template to automatically apply it, so that we can track the alumni PRs in an easier way.

[Kobzol]

@rustbot label +C-enhancement

(Just testing if triagebot works here)

[rustbot]

Error: The feature relabel is not enabled in this repository.
To enable it add its section in the triagebot.toml in the root of the repository.

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #triagebot on Zulip.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.