Add UDPRoute to VirtualService

[kirgene]

Adds a udp field on VirtualService and a UDPRoute message for L4 UDP routing. Mirrors the existing TCPRoute structure.

Used by istio/istio#59664 to carry Gateway API UDPRoute translations alongside HTTP, TCP, and TLS routes.

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[keithmattix]

This is a big step; Istio has never supported UDP service-to-service. We'd need a design doc to even begin to understand how this feature would work