Support mixed externalname and local services in single route

[ciarams87]

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Proposed changes

Problem: When routing to an ExternalName service alongside internal services in the same HTTPRoute, NGF incorrectly sets the Host header to the Gateway's hostname instead of the external service's hostname, causing requests to fail.

Solution: Properly set Host header based on ExternalName service hostname.

Testing: Local testing using the updated examples and unit testing

Please focus on (optional): If you any specific areas where you would like reviewers to focus their attention or provide specific feedback, add them here.

Closes #4187

Checklist

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• [ ] I have read the CONTRIBUTING doc
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have checked that all unit tests pass after adding my changes
• [ ] I have updated necessary documentation
• [ ] I have rebased my branch onto main
• [ ] I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

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Support mixed externalname and local services in single route.