id of instance templates now relies on the unique id of the resource

[modular-magician]

This is a security improvement to prevent instance templates being replaced after an instance template is created and before it is used by another resource (e.g. MIG creation). It introduces the self_link_unique computed attribute for google_compute_instance_template's that can be used to prevent TOCTOU attacks when resources are created in an untrusted environment.

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• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
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Release Note Template for Downstream PRs (will be copied)

compute: added output-only attribute `self_link_unique` id for `google_compute_instance_template` to point to the unique id of the resource (instead of its name). It is recommended to use `self_link_unique` for references over `self_link` and `id` for `google_compute_instance_template`

Derived from GoogleCloudPlatform/magic-modules#7358