Ensure cudf::ast::expressions api doesn't use detail types

[robertmaynard]

Description

The public api of cudf::ast::expression required using cudf::ast::detail types which goes against the cudf programming guide. So we move the required types to use cudf::ast::expression into the public stable API.

Checklist

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