Add EXTERNAL_MANAGED option to global forwarding rule and add example

[modular-magician]

This adds the EXTERNAL_MANAGED load balancing scheme option to the Global Forwarding Rule resource and adds an example that will generate tests.

This option was added in the DCL (the change was merged in a previous PR GoogleCloudPlatform/magic-modules#5586), but I'm also adding it here so that it automatically generates the test and examples.

Fixes hashicorp/terraform-provider-google#10858

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 `EXTERNAL_MANAGED` as option for `load_balancing_scheme` in `google_compute_global_forwarding_rule` resource

Derived from GoogleCloudPlatform/magic-modules#5611