feat(ingressnginx): implement x-forwarded-prefix annotation support

[asklokesh]

Summary

Implements support for header manipulation annotations as requested in #271.

Annotations addressed:

• nginx.ingress.kubernetes.io/x-forwarded-prefix - Implemented: Sets X-Forwarded-Prefix request header
• nginx.ingress.kubernetes.io/custom-headers - Improved warning: Explains that ConfigMap reading is not supported during migration
• nginx.ingress.kubernetes.io/connection-proxy-header - Already implemented (sets Connection header)
• nginx.ingress.kubernetes.io/upstream-vhost - Already implemented (sets Host header)

Implementation details:

• x-forwarded-prefix is skipped in headers.go when rewrite-target is also present (rewrite.go handles it in that case to avoid duplicate processing)
• Improved custom-headers warning message to guide users on manual migration
• Fixed typo in annotations.go comment

Changes:

• annotations.go: Fixed "R2gex" -> "Regex" typo
• headers.go: Added x-forwarded-prefix support with rewrite-target coordination, improved custom-headers warning
• headers_test.go: Added test cases for x-forwarded-prefix, skip-when-rewrite-present, and no-annotations edge cases

Fixes #271

Test plan

• Unit tests added and passing
• Verified x-forwarded-prefix header is set correctly
• Verified x-forwarded-prefix is not duplicated when rewrite-target is present
• All existing tests pass

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: asklokesh
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[linux-foundation-easycla]

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• ✅ login: asklokesh / name: Lokesh Mure (21dd3f1)

[k8s-ci-robot]

Hi @asklokesh. Thanks for your PR.

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[asklokesh]

/easycla

[asklokesh]

Closing this PR as header manipulation was already implemented and merged in PR #283 by @eladmotola, and additional header work is being done in PR #291 by @Stevenjin8. checklist item added to have checked for existing PRs before starting.