Generalize S3FileSystem

I'm working with multiple storage systems that speak the S3 api. I would like to support FileIO operations for these storage systems, but S3FileSystem hardcodes the s3 scheme (the various systems use different URI schemes) and it is in any case impossible to instantiate more than one in the current design.

I'd like to refactor the code in org.apache.beam.sdk.io.aws.s3 (and maybe ...aws.options) somewhat to enable this use-case. I haven't worked out the details yet, but it will take some thought to make this work in a non-hacky way.