Add via parameter on join room requests as per MSC4156

[Johennes]

This implements MSC4156 which will ship with Matrix 1.12.

Pull Request Checklist

• I have added Go unit tests or Complement integration tests 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.
• Pull request includes a sign off below using a legally identifiable name or I have already signed off privately

Signed-off-by: Johannes Marbach <[email protected]>

[S7evinK]

Thanks for contributing to Dendrite!

The repo has been cloned into element-hq/dendrite and made the switch to AGPL as per https://matrix.org/blog/2023/11/06/future-of-synapse-dendrite/.
For more information, feel free to read this and this

If you still wish to contribute under the new license, feel free to create a new PR over at element-hq/dendrite.