Temporary use forked prometheus/common to fix ProxyURL unmarshalling

[OlivierCazade]

I created a pull request in the prometheus/common repository:
prometheus/common#394

Meanwhile, this PR change the dependency to point to our local fork so we can marshall/unmarshall loki HTTP config without Proxy errors.

[codecov-commenter]

Codecov Report

Merging #263 (a5ef225) into main (467e409) will decrease coverage by 1.43%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #263      +/-   ##
==========================================
- Coverage   61.63%   60.19%   -1.44%     
==========================================
  Files          67       69       +2     
  Lines        3915     4095     +180     
==========================================
+ Hits         2413     2465      +52     
- Misses       1350     1469     +119     
- Partials      152      161       +9     

Flag	Coverage Δ
unittests	60.19% <ø> (-1.44%)	⬇️

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

Impacted Files	Coverage Δ
pkg/pipeline/write/write_stdout.go	59.37% <0.00%> (-40.63%)	⬇️
pkg/confgen/confgen.go	43.47% <0.00%> (-3.40%)	⬇️
pkg/test/utils.go	62.50% <0.00%> (-1.85%)	⬇️
pkg/pipeline/ingest/ingest_collector.go	48.12% <0.00%> (-1.51%)	⬇️
pkg/pipeline/ingest/ingest_kafka.go	80.28% <0.00%> (-0.49%)	⬇️
pkg/pipeline/encode/encode_kafka.go	69.01% <0.00%> (-0.35%)	⬇️
pkg/confgen/config.go	42.85% <0.00%> (ø)
pkg/api/encode_kafka.go	100.00% <0.00%> (ø)
cmd/confgenerator/main.go	0.00% <0.00%> (ø)
cmd/flowlogs-pipeline/main.go	0.00% <0.00%> (ø)
... and 4 more

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[jpinsonneau]

LGTM with prometheus/common@main...netobserv:prometheus-common:proxy-url-backport