Add via parameter on join room requests as per MSC4156

This implements [MSC4156](https://github.com/matrix-org/matrix-spec-proposals/pull/4156) which will ship with Matrix 1.12. ### Pull Request Checklist <!-- Please read https://matrix-org.github.io/dendrite/development/contributing before submitting your pull request --> * [ ] I have added Go unit tests or [Complement integration tests](https://github.com/matrix-org/complement) for this PR _or_ I have justified why this PR doesn't need tests The existing `joinroom_test.go` doesn't seem to assert on the `server_name` parameter. I'd be happy to add tests for both `server_name` and `via` but might need some guidance. * [x] Pull request includes a [sign off below using a legally identifiable name](https://matrix-org.github.io/dendrite/development/contributing#sign-off) _or_ I have already signed off privately Signed-off-by: `Johannes Marbach <n0-0ne+github@mailbox.org>`