[8.16](backport #6675) resolve proxy to inject using http package

[mergify]

What does this PR do?

Agent is injecting proxy env variables into components configs if no proxy is already specified by the config. Unfortunately with this approach the NO_PROXY variable is lost, so the target URL has to be evaluated against the environment proxy to determine if the proxy should be used. Go http provides such functionality.

Why is it important?

The environment proxy may be defined with exclusion for Fleet and Output addresses.

Checklist

• I have read and understood the pull request guidelines of this project.
• 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/fragments using the changelog tool
• I have added an integration test or an E2E test

Related issues

• Closes Injected proxy variables into endpoint elasticsearch does not include NO_PROXY variable #6209
• Relates Forward HTTP_PROXY/HTTPS_PROXY/NO_PROXY to components. #2602

---

This is an automatic backport of pull request #6675 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of be179d8 has failed:

On branch mergify/bp/8.16/pr-6675
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit be179d886.
  (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)

Changes to be committed:
	new file:   changelog/fragments/1738329703-environment-proxy-injection-fix.yaml
	modified:   internal/pkg/agent/application/inject_proxy_component_modifier.go
	modified:   internal/pkg/agent/application/inject_proxy_component_modifier_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   NOTICE.txt
	both modified:   go.mod

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

[mergify]

This pull request has not been merged yet. Could you please review and merge it @intxgo? 🙏

[pkoutsovasilis]

@intxgo could you please take care of this PR? The NOTICE.txt needs update

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
77.8% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube