Allow subclasses of S3Client, etc to set a custom scheme

[kujenga]

Currently, the guidance for accessing S3 compatible stores recommends continuing to use the S3 scheme in the path: https://cloudpathlib.drivendata.org/stable/authentication/#accessing-custom-s3-compatible-object-stores

I'd like to be able to take this a step further and customize the URL scheme. The following currently runs into an issue as cloud_prefix is not available on the client implementations, which instead hard-code their scheme.

@register_path_class("mys3")
class MyS3Path(S3Path):

    cloud_prefix: str = "mys3://"


@register_client_class("mys3")
class MyS3Client(S3Client):

    cloud_prefix: str = "mys3://"

The idea here is to allow the schemes to be customized in a similar way to how the Path implementations work:

cloudpathlib/cloudpathlib/s3/s3path.py

Line 28 in b776bee

cloud_prefix: str = "s3://"

[pjbull]

Closing as done in #467

[pjbull]

@kujenga Just released v0.20.0 to PyPI, which includes #467. Take it for a spin!