Acceptance criteria:
1. SMCP allows to configure Prometheus as an extension provider:

spec: 
  extensionProviders: 
  - name: prometheus
    prometheus: {}

2. Configured values are mapped 1:1 to meshConfig with helm values.

Testing:
When "prometheus" extension provider is configured:
1. telemetry.enabled should be true in the status.appliedValues.
2. Proxies should return metrics istio_requests_total for HTTP services and istio_tcp_connections_opened_total for TCP services.

Prometheus extension provider should be tested with disabled addons, in particular with disabled addons.prometheus, because this setting enables traffic metrics in istio-proxies and we want to check if extension provider enables these metrics.