Add login fallback

[S7evinK]

Part of #3216

The files are basically copied from Synapse, with minor changes to the called endpoints. We never seem to have had the /_matrix/static/client/login/ endpoint, this adds it.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (3a4b5f4) 65.57% compared to head (e2fb34e) 65.43%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
setup/base/base.go	50.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3302      +/-   ##
==========================================
- Coverage   65.57%   65.43%   -0.15%     
==========================================
  Files         509      509              
  Lines       57482    57488       +6     
==========================================
- Hits        37695    37616      -79     
- Misses      15926    15990      +64     
- Partials     3861     3882      +21     

Flag	Coverage Δ
unittests	50.01% <50.00%> (-0.03%)	⬇️

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