Add support for configurable routers and gateways

[anfredette]

The current deployment architecture assumes a specific routing/gateway setup. We should support multiple router/gateway options to match different production environments.

Acceptance Criteria

• Identify common router/gateway options (e.g., Istio, NGINX, Envoy, KServe predictor)
• Define configuration schema for router selection
• Create deployment templates for each router type
• Add UI option to select router/gateway during deployment
• Document routing architecture for each option
• Test with KV cache-aware routing (see item Add llm-d as deployment target alongside KServe/vLLM #11 in work items)

Notes

• Routing impacts latency, throughput, and KV cache efficiency
• Should support KV cache-aware routing for multi-replica deployments (llm-d feature)