Google cloud storage compatability

[tomlarkworthy]

I love this lib. I have tested it successfully with S3, minio, backblaze and R2. However, I am getting stuck with GCS. I have not worked out the root problem yet, I suspect GCS requires signed content-lengh headers or something, which is explicitly turned off in this library (for proxy support?)
Just wondering how open you are for a PR in this direction, I think GCS XML might work too differently to the others, do you want to support more vendors?
I think if I were to try and fix it I would declare the service "GCS" instead of "S3" and use that as the hook for feature flags. Wdyt?