Add via parameter on join room requests as per MSC4156

[Johennes]

This implements MSC4156 which will ship with Matrix 1.12 and is the continuation of matrix-org/dendrite#3438.

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
• Pull request includes a sign off below using a legally identifiable name or I have already signed off privately

Signed-off-by: Johannes Marbach n0-0ne+github@mailbox.org

[CLAassistant]

All committers have signed the CLA.

[S7evinK]

LGTM, thanks!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 30.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 49.40%. Comparing base (7cc7ebb) to head (f5e5396).

Files with missing lines	Patch %	Lines
clientapi/routing/joinroom.go	30.00%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3441      +/-   ##
==========================================
+ Coverage   49.36%   49.40%   +0.04%     
==========================================
  Files         522      522              
  Lines       59465    59472       +7     
==========================================
+ Hits        29353    29383      +30     
+ Misses      26647    26632      -15     
+ Partials     3465     3457       -8     

Flag	Coverage Δ
unittests	49.40% <30.00%> (+0.04%)	⬆️

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