gateway: add configurable response write timeout

[gammazero]

Closes #679
Replacement for #812

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 60.40%. Comparing base (b0125d1) to head (0b98918).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
gateway/handler.go	77.77%	2 Missing ⚠️

@@            Coverage Diff             @@
##             main     #818      +/-   ##
==========================================
- Coverage   60.48%   60.40%   -0.08%     
==========================================
  Files         244      244              
  Lines       31100    31108       +8     
==========================================
- Hits        18811    18791      -20     
- Misses      10611    10634      +23     
- Partials     1678     1683       +5     

Files with missing lines	Coverage Δ
gateway/gateway.go	83.54% <ø> (ø)
gateway/handler.go	76.25% <77.77%> (-0.02%)	⬇️

... and 13 files with indirect coverage changes

[hsanjuan]

This doesn't act as nginx's proxy_read_timeout though?... i.e. a roundtrip should not be killed just because it took 30 seconds... the problem is 30 seconds without writing any data on the response body.

Instead of #812, which goes in that direction, perhaps we can get away with it by setting ResponseHeaderTimeout in the client configuration. Normally if a request lasts more than 30 seconds without any data coming back, the reason is that we are still looking for the content, and I doubt we have written any response headers by then.