Add Google Cloud Run worker identity/deployment helper

[seanbollin]

What & why

The .NET SDK already ships an AWS Lambda extension (Temporalio.Extensions.Aws.Lambda) that sets the worker "ID" for Lambda, and a committed Temporalio.Extensions.Gcp.CloudRun.OpenTelemetry package for Cloud Run observability. This adds the worker-identity / deployment-version piece for Google Cloud Run, as the base Temporalio.Extensions.Gcp.CloudRun package, covering both Cloud Run worker pools and services.

Cloud Run runs a long-lived container, so — unlike Lambda's per-invocation handler — there is nothing to wrap. This is a small metadata helper that follows the same idiom as the sibling .OpenTelemetry package (GoogleCloudRun-prefixed types, ApplyGoogleCloudRun… extension methods, WARNING: … experimental remark).

What it does

GoogleCloudRunMetadata.FetchAsync(), called once at startup:

• resolves the deployment Name from CLOUD_RUN_WORKER_POOL (worker pools) → K_SERVICE (services), and the Revision from CLOUD_RUN_REVISION → K_REVISION, and
• makes a single HTTP GET to the instance metadata server (http://metadata.google.internal/computeMetadata/v1/instance/id, header Metadata-Flavor: Google) for the unique instance id (available on both).

Extension methods apply it to your normal, long-lived worker:

• await connectOptions.ApplyGoogleCloudRunDefaultsAsync() → fetches metadata, sets Identity to {InstanceId}@{Revision} (only if unset), and returns the metadata; and
• workerOptions.ApplyGoogleCloudRunDefaults(metadata) → sets DeploymentOptions = new WorkerDeploymentOptions(metadata.ToWorkerDeploymentVersion(), useWorkerVersioning: true).

(WorkerIdentity / ToWorkerDeploymentVersion() are also exposed for direct use.)

Notes

• Draft — opening early for design feedback. Idiom intentionally matches the committed sibling Temporalio.Extensions.Gcp.CloudRun.OpenTelemetry.
• No new package dependencies (HttpClient is in the BCL).
• No unit tests yet (kept intentionally minimal for the draft).
• Follow-up before un-drafting: wire the new package into .github/workflows/nuget-package.yml and the API-docs (src/Temporalio.ApiDoc/…) the way the .OpenTelemetry package is, and add tests under tests/Temporalio.Tests/Extensions/Gcp/CloudRun/.
• Verified locally: warning-clean dotnet build (TreatWarningsAsErrors + StyleCop + full analyzers), dotnet format --verify-no-changes, and dotnet pack all pass.

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