Add instrumentation for `google-cloud-storage`

[basepi]

Because it's using REST APIs underneath, spans are currently being created. But there are (at least) two issues:

1. No enrichment with storage-related context like we do with botocore for s3
2. Any .exists checks set the span status to failure because it can return a 404 if the file doesn't exist.

A first-class instrumentation would be a much better experience.

Ref https://github.com/elastic/apm-agent-python/issues/1760