[9.0](backport #46791) Add proxy tests to beatsauth extension

[mergify]

Proposed commit message

This PR adds proxy related tests to beatsauth extension

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

cd libbeat/otelbeat/oteltests/
go test -run TestProxyHTTP -v

Related issues

• Related #45491

---

This is an automatic backport of pull request #46791 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of b5c5158 has failed:

On branch mergify/bp/9.0/pr-46791
Your branch is up to date with 'origin/9.0'.

You are currently cherry-picking commit b5c515868.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   NOTICE.txt
	both modified:   go.mod
	both modified:   go.sum
	both modified:   libbeat/otelbeat/beatconverter/beatconverter.go
	both modified:   libbeat/otelbeat/beatconverter/beatconverter_test.go
	deleted by us:   libbeat/otelbeat/oteltest/beatsauth_test.go

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)