Adding support for traffic shaping for more than one alternative backends

[mavriq]

What this PR does / why we need it:

This PR enhances the ingress-nginx balancer functionality by adding support for multiple alternative backends.
Currently, the balancer can only use one alternative backend, but this change allows defining multiple alternative backends and selecting the first suitable one based on request context.

Types of changes

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Updated existing unit tests in balancer_test.lua to cover the new functionality
• Added comprehensive tests for multiple alternative backends scenarios
• Verified that all existing tests pass with the modified implementation
• Tested various scenarios including canary deployments with cookies, headers, and weights

Checklist:

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

Key Changes:

• Replaced route_to_alternative_balancer with is_alternative_backend_suitable and get_alternative_or_original_balancer
• Added support for multiple alternative backends in alternative_backends list
• Maintained backward compatibility with existing configurations
• Simplified the get_balancer function logic

[netlify]

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[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: mavriq / name: mavriq (1dd7925, 4ffb007, 65c8a5f, 7205680, 99abd25)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mavriq
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[k8s-ci-robot]

Hi @mavriq. Thanks for your PR.

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