Add support in compute global forwarding rules for service directory registrations

[googlerjk]

Added support in compute global forwarding rules for service directory registrations

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

• [x] 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).
• [x] Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• [x] 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).
• [x] Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• [x] Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

Added `service_directory_registrations` support to `forwarding_rule` resource.