Resolve prom-label-proxy gateway class and cert from the cluster

[ArnobKumarSaha]

prom-label-proxy assumed the GatewayClass and the <ns>-gw-cert secret are named after its own namespace. That's true on a monitoring cluster but not on the ACE hub.

• New helper prom-label-proxy.gatewayClass: returns the namespace if a GatewayClass with that name exists, otherwise the GatewayClass annotated catalog.appscode.com/is-default-gatewayclass: "true" (found via lookup).
• Used for gatewayClassName, listener certificateRefs, BackendTLSPolicy caCertificateRefs, and the proxy-tls volume.
• Monitoring cluster: unchanged (monitoring / monitoring-gw-cert). Hub: ace / ace-gw-cert, which config-syncer copies into monitoring.
• Offline helm template falls back to the namespace; output is identical to master.

Depends on: $U1

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