[FEA] simplify `page_state_s` and possibly other structures for specialized parquet kernels

@vuule brought up the point here https://github.com/rapidsai/cudf/pull/15159#discussion_r1505126922 that `page_state_s` could be simplified for the fixed/fixed dictionary kernels, as they don't need all the members of this structure and it might help with shared memory use.

I also think there are other shared structures that we should audit and clean up. I am filing this issue so we don't loose track of it.