Add addressing_style option for S3 virtual-hosted-style custom endpoints

S3 providers like CoreWeave (cwobject.com) only support virtual addressing, and reject path-style requests. fsspec for instance enables this when you configure it with `{"s3": {"addressing_style": "virtual"}}` This adds an "addressing_style" option ("path" or "virtual") to the S3 kvstore driver spec, enabling virtual-hosted-style URL construction ({bucket}.{endpoint_host}) for custom endpoints. Default behavior is unchanged (path-style for custom endpoints). Fixes: marin-community/marin#2938 (N.B. I have approximately zero knowledge about this stuff).