Allow summing of Decimal128 types

[jaychia]

Is your feature request related to a problem? Please describe.

Decimal128 should support the summation aggregation

import daft

df = daft.from_pydict({"foo": [1, 1, 2, 2, 3, 3]})
df = df.with_column("foo_decimal", df["foo"].cast(daft.DataType.decimal128(7, 2)))
df = df.sum("foo_decimal")

df.collect()