Add Service Attachment support for Secure Web Proxy

[modular-magician]

This PR adds the support for Secure Web Proxy (Gateway) on the Service Attachment resource.
This is provided on the field _ targetService_ which now will have support for the Forwarding Rule and the Secure Web Proxy (Gateway).

Note 1: Test scenario was needed to verify if it is working with SWP.

Note 2: Diff Suppress function was needed because the fields maybe be slightly different when using self_link and this will generate a change on the resource and break the test. For example:

OLD = "https://networkservices.googleapis.com/beta/projects/{PROJECT_ID}/locations/us-east1/gateways/{SWP_ID}"
NEW = "https://networkservices.googleapis.com/v1alpha1/projects/{PROJECT_ID}/locations/us-east1/gateways/{SWP_ID}"

Release Note Template for Downstream PRs (will be copied)

compute: changed `target_service` field on the `google_compute_service_attachment` resource to accept a `ForwardingRule` or `Gateway` URL.

Derived from GoogleCloudPlatform/magic-modules#11107

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