Reuse prefetched parquet metadata for decimal predicate pushdown

We have the schema in C++ https://github.com/rapidsai/cudf/blob/bc6d10cb927cc0f804834c53790faf56f055faeb/cpp/include/cudf/io/parquet_schema.hpp#L869. The issue is that we need python bindings. And a small utility to convert `std::vector` to parquet schema. The logic I added in https://github.com/rapidsai/cudf/pull/21458 can probably be hoisted into said utility.

_Originally posted by @Matt711 in https://github.com/rapidsai/cudf/pull/22700#discussion_r3375319611_