Add Agentgateway Emitter and Port Kgateway Emitter Features

[danehans]

AgentgatewayPolicy Mapping

Traffic (spec.traffic)

• CORS → traffic.cors (similar structure to Gateway API HTTPCORSFilter)
• Rate limiting → traffic.rateLimit.local (LocalRateLimit with requests/tokens per unit)
• Timeouts → traffic.timeouts.request (proxy-send-timeout) and traffic.timeouts for stream idle
• Basic auth → traffic.basicAuthentication (with users or secretRef)
• External auth → traffic.extAuth (direct backendRef, no GatewayExtension needed)
• Rewrite target → traffic.transformation.request (path rewrite)

Backend (spec.backend)

• Backend TLS → backend.tls (mtlsCertificateRef, caCertificateRefs, insecureSkipVerify, sni, verifySubjectAltNames)
• Backend connect timeout → backend.tcp.connectTimeout
• Backend protocol → backend.http.version (HTTP1/HTTP2) or handled via AgentgatewayBackend

Frontend (spec.frontend)

• Access logging → frontend.accessLog (with filter and attributes)
• TLS settings → frontend.tls (handshakeTimeout, alpnProtocols)
• HTTP settings → frontend.http (http1/http2 settings)
• Buffer/body size → frontend.http (maxBufferSize)

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AgentgatewayBackend Mapping

• Service upstream → AgentgatewayBackend with spec.static (host, port)
• Backend protocol → AgentgatewayBackend.spec.static doesn't have appProtocol, but can use backend.http.version in policy

[danehans]

Reopening since #63 only partially fixes this issue.

[danehans]

xref kgateway-dev/kgateway.dev#623 for user docs.