Add extended CONNECT setting to ProxyConfig

[mpuncel]

🤖 Draft API change for configuring HTTP/2 extended CONNECT through an Istio CRD instead of proxy environment metadata.

Summary:

• Adds nullable enableHttp2Connect to networking.istio.io/v1beta1 ProxyConfig.
• Adds matching enable_http2_connect to mesh ProxyConfig so the CRD can map into effective proxy configuration.
• Documents that the setting affects sidecar listener HTTP connection managers and does not configure upstream clusters.
• Regenerates Go, HTML docs, and CRD schema outputs.

Validation:

• ./gen.sh
• go test ./...
• git diff --check