CI: add e2e test coverage report

[OverOrion]

Is your feature request related to a problem? Please describe.
It would be great to have the e2e tests coverage reports as they cover much a bigger part of the code base compared to unit tests.
Some resources:

• https://go.dev/blog/integration-test-coverage
• https://go.dev/testing/coverage/

Describe the solution you'd like
Build the binary with -cover flag, run the binary with GOCOVERDIR=some_coverage_dir and at the end of the e2e test run merge those files to have a nice (hopefully) high overall coverage report.

Describe alternatives you've considered
I do not think there is any.

Additional context

The tricky part is getting those reports, because currently the CI runs the e2e tests using kind. As mentioned in the docs graceful shutdown is needed for the program to write it's report, so a simple kubectl cp command might not be enough (not tested, just a theory).

[OverOrion]

Just a rather drafty branch: https://github.com/kube-logging/logging-operator/tree/gha-add-go-test-coverage_e2e

[OverOrion]

kudo's to pepov for finding this:
https://github.com/GoogleContainerTools/skaffold/blob/main/examples/go-integration-coverage/skaffold.yaml#L46C2-L49C31