[FEA] Deduplicate fixed-width plain parquet decoder code

We currently have two mostly-identical implementations of the plain decoder kernel for fixed width types in parquet, one in `decode_fixed.cu` and the other in `dictionary_page_filter.cu`. We should refactor and deduplicate them to use the same functions at both places.

_Originally posted by @nvdbaranec in https://github.com/rapidsai/cudf/pull/18836#discussion_r2167391721_