support `as_of` parameter in `read_pandas_sql`

[kretes]

This is a feature request for adding as_of parameter to read_pandas_sql. This parameter is available in read_pandas for reading the whole table, but it would be nice to have the same in a generic sql-based method: https://github.com/dolthub/doltpy/blob/main/doltpy/cli/read.py#L23