Add instrumentation for `google-cloud-storage`

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](https://github.com/elastic/apm-agent-python/blob/main/elasticapm/instrumentation/packages/botocore.py)
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