[PROPOSAL] Define fixed-point types in `cudf::` namespace

Fixed-point (decimal) types are defined in a top-level namespace `numeric` and not in the namespace `cudf`.

This shows up in the stringified type name, and violates the expectation that packages should probably keep all their definitions in a namespace owned by the package, i.e. `cudf::`.

See additional discussion: https://github.com/rapidsai/cudf/pull/12158#discussion_r1023411485

I propose moving the fixed-point code to a namespace `cudf::fixed_point` and refactoring as needed.