[SPARK-11007] [SQL] Adds dictionary aware Parquet decimal converters

For Parquet decimal columns that are encoded using plain-dictionary encoding, we can make the upper level converter aware of the dictionary, so that we can pre-instantiate all the decimals to avoid duplicated instantiation.

Note that plain-dictionary encoding isn't available for `FIXED_LEN_BYTE_ARRAY` for Parquet writer version `PARQUET_1_0`. So currently only decimals written as `INT32` and `INT64` can benefit from this optimization.

Author: Cheng Lian <lian@databricks.com>

Closes #9040 from liancheng/spark-11007.decimal-converter-dict-support.