Add new auth headers required for tailing k6 Cloud logs

[federicotdn]

What?

Adds two new HTTP headers required to tail logs from k6 Cloud in the near future.

Why?

See issue #3216 and backlinked issue from the Grafana private repository.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make ci-like-lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

#3216

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #3224 (dbd85f2) into master (59d9f9b) will decrease coverage by 0.02%.
The diff coverage is 96.22%.

❗ Current head dbd85f2 differs from pull request most recent head e80b072. Consider uploading reports for the commit e80b072 to get more accurate results

@@            Coverage Diff             @@
##           master    #3224      +/-   ##
==========================================
- Coverage   73.18%   73.17%   -0.02%     
==========================================
  Files         259      257       -2     
  Lines       19864    19886      +22     
==========================================
+ Hits        14538    14551      +13     
- Misses       4404     4408       +4     
- Partials      922      927       +5     

Flag	Coverage Δ
ubuntu	73.17% <96.22%> (+0.04%)	⬆️
windows	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
output/cloud/expv2/flush.go	93.40% <94.59%> (+2.36%)	⬆️
cloudapi/logs.go	85.29% <100.00%> (+0.29%)	⬆️
output/cloud/expv2/output.go	74.23% <100.00%> (+1.39%)	⬆️

... and 8 files with indirect coverage changes