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.

[vyasr]

@robertmaynard does this need to get in anytime soon?

[robertmaynard]

Lower priority now that we have marked all the detail namespace as exported. But still weird that cudf has public stable API that requires detail types.

[vyasr]

I think we ought to rethink what interfaces are public and which ones are private. I think there's a lot of APIs that are marked as public because they need to be used by other parts of the code within libcudf, but they're not really intended for end-users.

[davidwendt]

I'm also starting to see a possible need for a public-ish set of classes/functions for users to build custom kernels or UDFs which may also relate to AST expressions too.