Add via parameter for MSC4156

[Johennes]

This is the client-side implementation for https://github.com/matrix-org/matrix-spec-proposals/pull/4156.

Checklist

• [ ] Tests written for new code (and old code if feasible).
• [ ] New or updated public/exported symbols have accurate TSDoc documentation.
• [ ] Linter and other CI checks pass.
• [x] Sign-off given on the changes (see CONTRIBUTING.md).

[Johennes]

Can we not just be lazy

or always include both parameters

I thought I wouldn't pass review with that. 😅

But yes, if you're ok with this, it'll make things a bit less verbose.