Add support for gzip in csv output format

[thejasbabu]

Fixes: #1550

Refactor:

• Add closeFn for CSV collector
• Made writeToFile protected

Tested this by using the following script

test.js

import http from 'k6/http';
import { sleep } from 'k6';

export default function () {
  http.get('http://test.k6.io');
  sleep(1);
}

and running

./k6 run --out csv=test.csv.gz test.js

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #1566 into master will increase coverage by 0.01%.
The diff coverage is 93.10%.

@@            Coverage Diff             @@
##           master    #1566      +/-   ##
==========================================
+ Coverage   77.11%   77.12%   +0.01%     
==========================================
  Files         162      162              
  Lines       13192    13204      +12     
==========================================
+ Hits        10173    10184      +11     
- Misses       2499     2500       +1     
  Partials      520      520              

Impacted Files	Coverage Δ
stats/csv/collector.go	86.17% <93.10%> (+2.39%)	⬆️
lib/testutils/minirunner/minirunner.go	78.57% <0.00%> (-3.58%)	⬇️
lib/executor/vu_handle.go	95.49% <0.00%> (-0.91%)	⬇️
lib/executor/constant_arrival_rate.go	96.75% <0.00%> (-0.55%)	⬇️
stats/cloud/collector.go	79.04% <0.00%> (+0.59%)	⬆️

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[thejasbabu]

Added document for CSV format here

[imiric]

Hey, thanks for your work @thejasbabu!

This looks good besides the two minor comments. We're aiming for this to be part of v0.28.0 👍

[imiric]

Thank you for this and the additional test!