Redesign ServiceExport spec

[solmonk]

An extension of #365

The controller is not yet allowing exporting TG for use in routes other than HTTPRoute. As more route types (TLSRoute, TCPRoutes, and more) are coming, we need a fundamental solution to support all route types.

An original suggestion was to create all types of target groups when exported - but it is technically a waste of resource, and actually not compatible with common use cases.

For example, let's say a service has 2 exported ports - 80 serves plaintext HTTP, and 8081 serves GRPC.

To support all types of traffic we need 2 different types of target groups to be attached to 2 different routes.

Now that we have our own ServiceExport spec, we could define something like this on ServiceExport, and be explicit on which target groups are to be exported:

spec:
  exportedPorts:
  - port: 80 # this is equivalent to backendref port
    routeType: HTTP
  - port: 8081
    routeType: GRPC

The following behavior is required:

• For each exported port, create target group to be used with relevant route type
• When importing service, backendRef port must match the exported port.
• For backwards compatibility, keep the old behavior when no exportedPort is defined (export all ports to HTTP target group.)