WIP: Improve cudf::io::datasource::create().

[tpn]

This commit introduces the new datasource_kind and datasource_params data types, and updates the cudf::io::datasource::create() signature to allow parameterized datasource creation.

N.B. Non-compiling checkpoint commit.

Description

Checklist

• [ ] I am familiar with the Contributing Guidelines.
• [ ] New or existing tests cover these changes.
• [ ] The documentation is up to date with these changes.