GEP-1748: Use ServiceExport instead of ServiceImport as parentRef

[costinm]

One of the main improvements in Gateway API is separating the 'frontend' and 'backend' roles. ServiceImport fits very well as a 'backend' - as used in the examples in GEP 1748.

However in 'mesh' use cases it may be more intuitive and clear to have the ServiceExport as a parent.

An additional benefit: ServiceExport is created by a user ( usually with RBAC permissions) and represent the intent to have a service available in all clusters. ServiceImport is usually created by an automation tool / MCS implementation. For a user to create a proper config with helm or kubeconfig they would need to wait for the ServiceImport to show up ( or attempt to create one directly).

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