Add GCP resource detector

[dashpole]

This implements resource detection for GCE, GKE, Cloud Run, Cloud Functions, and App Engine. It doesn't split out logic into a non-otel package, which can be done as a follow-up.

Breaking changes from the opentelemetry-go-contrib gcp detector:

• Detectors for each platform are combined into a single detector
• No longer supports detecting k8s.pod.name, container.name, k8s.namespace.name. These are expected to be set outside of this resource detector.
• No longer sets service.namespace to "cloud-run-managed"
• Sets faas.name instead of service.name in cloud run
• Sets faas.id instead of service.instance.id in cloud run

Additions:

• Adds detector for Google App Engine
• Now correctly sets cloud.platform on GKE and Cloud Run
• Now detects cloud.availability_zone/region when running on GKE with workload identity enabled (the GCE metadata server is not accessible in that scenario)
• Now detects faas.version on cloud run and cloud functions.

[dashpole]

I raised this at the Go sig. I'll plan to split this between our repo, and upstream. This repo should not depend on semconv.

[dashpole]

Closing in favor of #394