test(customheaders): add test for annotation added

[qvalentin]

What this PR does / why we need it:

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] CVE Report (Scanner found CVE and adding report)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation only

Which issue/s this PR fixes

fixes https://github.com/kubernetes/ingress-nginx/issues/11680. reproduces the issue with a e2e test. Based on https://github.com/kubernetes/ingress-nginx/pull/11709. I just added the e2e test.

How Has This Been Tested?

The existing e2e did only test the customheaders annotation on an newly created ingress. This pr adds a second e2e tests that tests what happens, when the annotation is added to an existing ingress.

Checklist:

• [ ] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have added unit and/or e2e tests to cover my changes.
• [ ] All new and existing tests passed.