support `as_of` parameter in `read_pandas_sql`

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

Start in doltpy/cli/read.py around line 23 and compare the existing read_pandas entry point with read_pandas_sql. Trace how the current method accepts and forwards query options, then confirm that read_pandas_sql supports the same as_of behavior as read_pandas; the issue is done when callers can supply that parameter to the generic SQL-based method.

Written by the indexing model from the issue text.