Add support for dtypes in merge upsert

[ogescalante]

Hey guys, I'm using the functionality of merge upsert and I'm having to make temporary parquet to parse dtypes before merging, or use the as_type function of pandas, but in the to_parquet function it already can cast the dtypes.
It would be awesome if you guys added the option to pass the dtypes just like the to_parquet function, and behind the scenes cast the dtypes before merging the datasets.