msggen: add multiwithdraw method

[daywalker90]

nothing special, adds:

• multiwithdraw

[cdecker]

These PRs would be much simpler to review and merge if they weren't this disjoint by the way. The way you are doing it at the moment, the .msggen.json gets reset inbetween runs, which can cause the CI to fail on master and we have to go back and refill them in.

Can you maybe create 5-method chunks? That'd also cut down on the reviewing overhead (I get notified for every single update to any of these PRs, which de-sensitivizes reviewers to notifications, and stuff gets lost in the torrent of notifications).

[cdecker]

Keep the changes to one method per commit, even when clustering, otherwise we lose the nice collation of manual changes and derived changes.

[cdecker]

Also, CI resources aren't free, so let's try to preserve them as much as possible, even if it is GH footing the bill, we should be respectful.