Helm: Restructure Istio Provider

[danehans]

Istio should be scoped within provider like other providers, ie.. gke:

Current:

# Options: ["gke", "istio", "none"]
provider:
  name: none

  # GKE-specific configuration.
  # This block is only used if name is "gke".
  gke:
    # Set to true if the cluster is an Autopilot cluster.
    autopilot: false

istio:
  destinationRule:
    # Provide a way to override the default calculated host
    host: "" 
    # Optional: Enables customization of the traffic policy
    trafficPolicy: {}
      # connectionPool:
      #   http:
      #     maxRequestsPerConnection: 256000

Suggested:

# Options: ["gke", "istio", "none"]
provider:
  name: none

  # GKE-specific configuration.
  # This block is only used if name is "gke".
  gke:
    # Set to true if the cluster is an Autopilot cluster.
    autopilot: false
  # This block is only used if name is "istio".
  istio:
    destinationRule:
      # Provide a way to override the default calculated host
      host: "" 
      # Optional: Enables customization of the traffic policy
      trafficPolicy: {}
        # connectionPool:
        #   http:
        #     maxRequestsPerConnection: 256000

The change should be backwards compatible to support user migration.