Add Ci workflow to compute code coverage

[nezdolik]

Workflow computes code coverage for cilium-proxy source files using custom report generator. It relies heavily on bazel remote caching stored in gcs bucket and all reports (per PR) are uploaded to another gcs bucket and stored for 7 days.
Sample report: https://storage.googleapis.com/cilium-proxy-coverage-reports/c08c59a/coverage/html/index.html

[nezdolik]

looks like secret is not propagated to my fork workflow, synced offline with @sayboras on how to fix it

[jrajahalme]

There is some back-and-forth between adding/commenting/updating the GH workflow, could you clean up the PR by squashing all or most of that?

[sayboras]

looks like secret is not propagated to my fork workflow

I think with the pull request target setup, the secrets will not be visible in forked repo. Once you clean up the commit, I can create another temp branch to test it out.

[nezdolik]

From offline discussion with @jrajahalme this feedback needs to be addressed:

Not sure I understand how this works, but I would expect that building test deps (//tests/...) before running the coverage build would not count the coverage on the pre-build test code, or will it be built again due to different compiler flags?

[nezdolik]

@sayboras are you able to create a test branch for this PR?

[sayboras]

are you able to create a test branch for this PR?

Sure, however, check format seems failing.

[nezdolik]

are you able to create a test branch for this PR?

Sure, however, check format seems failing.

let me fix it

[nezdolik]

@sayboras format is fixed

[nezdolik]

oh, this should be part of cilium/proxy .bazelrc file