Implement `KonnectGatewayControlPlane` reference in `KonnectExtension`

[mlavacca]

Problem Statement

The KonnectExtension can be used to customize DataPlane behavior for Konnect. the KonnectExtension needs a CP reference to properly work, and such a reference can be:

• the CP konnectID, or
• the KonnectGatewayControlPlane namespacedName

So far, only the first approach has been implemented, so we need to allow the KonnectExtension to be a KonnectGatewayControlPlaneRef as well.

Acceptance Criteria

• [ ] As a user, I can create a KonnectGatewayControlPlane in my cluster and use it to configure my DataPlane for Konnect through a properly configured KonnectExtension resource.
• [ ] Guide in https://docs.konghq.com/gateway-operator/latest/get-started/kic/create-gateway/ is updated to use the extension

[pmalek]

Added an AC to update the guide at https://docs.konghq.com/gateway-operator/latest/get-started/kic/create-gateway/ when implemented