Allow referencing IPs by reference in google_compute_forwarding_rule

[modular-magician]

Part of hashicorp/terraform-provider-google#8454

This API only accepts a reference under some circumstances, but returns the raw IP. This adopts the strategy used in google_compute_global_forwarding_rule in hashicorp/terraform-provider-google#8018 to resolve that issue. Thinking about it, I don't love the solution- if a user changes the address they're referencing, Terraform won't see a diff- but it's consistent and means if we fix that issue with GFR we can apply the same fix to FR.

I removed the custom Terraform documentation for each field (adding a note on top of the MMv1 docs) and updated the MMv1 docs at the same time, mostly removing the rules about what IP can be used where & linking the canonical source instead.

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• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• 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 the ability to specify `google_compute_forwarding_rule.ip_address` by a reference in addition to raw IP address

Derived from GoogleCloudPlatform/magic-modules#4673