Allow referencing IPs by reference in google_compute_forwarding_rule

[rileykarson]

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.

If this PR is for Terraform, I acknowledge that I have:

• 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).
• 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).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

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

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 5 files changed, 78 insertions(+), 97 deletions(-))
Terraform Beta: Diff ( 5 files changed, 78 insertions(+), 97 deletions(-))
Inspec: Diff ( 2 files changed, 2 insertions(+), 2 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=181255"

[rileykarson]

VCR failed due to infrastructure reasons, but tests at https://ci-oss.hashicorp.engineering/buildConfiguration/GoogleCloudBeta_ProviderGoogleCloudBetaMmUpstream/181270?buildTab=overview

[melinath]

LGTM - seems like a reasonable standardization.