This change is intended to make the default client implementations
more flexible so that their scheme can be customized. This can be
useful in scenarios where a subclass wants to implement a custom
scheme on e.g. a S3 compatible API [1] but with a custom scheme
so that the default S3 access is still also available.

[1] https://cloudpathlib.drivendata.org/stable/authentication/#accessing-custom-s3-compatible-object-stores

The tests have been updated to include a new s3-like rig which uses the
new scheme override functionality.

This isolates the implementation in response to PR feedback:
drivendataorg#467 (comment)