feat(proxy): support onResponse callback

[enkot]

Resolves #354

This PR adds onResponse callback to sendProxy and proxyRequest utilities.
It accepts proxy event as argument which can be used to modify the response headers, cookies etc.:

return proxyRequest(event, target, {
  onResponse(_event) {
    setHeader(_event, 'foo', 'bar')
  }
})

It can also be async:

return proxyRequest(event, target, {
  async onResponse(_event) {
    // some async stuff
  }
})

[enkot]

@pi0 Can we merge this one? It doesn't make any breaking change.

[pi0]

It would be also nice to pass over the actual response as second argument

[enkot]

Good idea, done!

[codecov]

Codecov Report

Merging #368 (e0dfee4) into main (03243ba) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #368      +/-   ##
==========================================
+ Coverage   74.80%   74.85%   +0.05%     
==========================================
  Files          26       26              
  Lines        2409     2414       +5     
  Branches      383      385       +2     
==========================================
+ Hits         1802     1807       +5     
  Misses        607      607              

Impacted Files	Coverage Δ
src/utils/proxy.ts	82.81% <100.00%> (+0.45%)	⬆️

[enkot]

Hi @pi0,
It would be great to merge this one. Or some more work should be done here?

[pavloniym]

Waiting for merge! This feature could potentially help to fix unjs/nitro#1237 and #376