[FEA] Deduplicate fixed-width plain parquet decoder code

[mhaseeb123]

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 #18836 (comment)