Support HTTPRoute referencing Service

[haiyanmeng]

What would you like to be added:
Currently, our controller only supports HTTPRoute referencing XBackend resources. The controller does not support HTTPRoute referencing Service resources. We should add support for this.

If the Service resource referenced in the HTTPRoute resource has a corresponding XBackend resource, and an XAccessPolicy resource targeting the XBackend resource, but the XBackend resource is not referenced by any HTTPRoute resource, the rules defined in the XAccessPolicy resource should not impact the route for the Service resource. The controller should update the status of the XAccessPolicy resource to indicate that the rules in the XAccessPolicy resource are not taking effect because the route is missing.

Why this is needed:
The Gateway conformance test expects an implementation to support this.

[haiyanmeng]

/assign @chuangw6

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In response to this:

/assign @chuangw6

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[chuangw6]

#138 has already implemented this.