[ENH] Add support for Decimal types

[randerzander]

A simple test of querying data which has decimal columns fails:

>>> import cudf
>>> df  = cudf.DataFrame({"id": [0, 1, 2]}
>>> ddf = dd.from_cudf(df, npartitions=1)
>>> ddf["id"] = ddf["id"].astype(Decimal64Dtype(10, 2))

>>> from dask_sql import Context
>>> c = Context()
>>> c.create_table("ddf", ddf)
>>> c.sql("select sum(id) from ddf").compute()

Trace:

Traceback (most recent call last):
  File "<input>", line 1, in <module>
    c.sql("select sum(id) from ddf").compute()
  File "/raid/rgelhausen/conda/envs/pynds3/lib/python3.10/site-packages/dask_sql/context.py", line 515, in sql
    rel, _ = self._get_ral(sql)
  File "/raid/rgelhausen/conda/envs/pynds3/lib/python3.10/site-packages/dask_sql/context.py", line 817, in _get_ral
    schemas = self._prepare_schemas()
  File "/raid/rgelhausen/conda/envs/pynds3/lib/python3.10/site-packages/dask_sql/context.py", line 777, in _prepare_schemas
    sql_data_type = python_to_sql_type(data_type)
  File "/raid/rgelhausen/conda/envs/pynds3/lib/python3.10/site-packages/dask_sql/mappings.py", line 113, in python_to_sql_type
    raise NotImplementedError(
NotImplementedError: The python type decimal64 is not implemented (yet)