Add traffic status field to cloud run v1 service.

[modular-magician]

This is an output-only field so it's not generally important for the managed resource, but it can be useful in the data source to get the current traffic.

Fixes hashicorp/terraform-provider-google#15153

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).
• 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).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read Write release notes before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

cloud-run: added `status.traffic` output fields to `google_cloud_run_service` resource 

Derived from GoogleCloudPlatform/magic-modules#8410