support `no_sign_request` for GCS

[geospatial-jeff]

This currently isn't supported.

async_tiff.exceptions.UnknownConfigurationKeyError: Configuration key: 'skip_signature' is not valid for store 'GCS'.

Debug source:
UnknownConfigurationKey {
    store: "GCS",
    key: "skip_signature",
}

I'd imagine it would be similar to developmentseed/obstore#404. Not super high priority as you can access any public GCS bucket through the S3 API:

aws s3 ls s3://ftw-aef-export/ --endpoint-url https://storage.googleapis.com --no-sign-request

[kylebarron]

If you update to latest object_store that'll work