Asyncpg's fetchmany() is not exposed in any QuartDB methods #29

Hi,

I need to access asyncpg's fetchmany() functionality. That is, a fetchall() but providing the method with a list of ValueObjects rather than just one. This is supported in the execute_many() method for a regular Connection, which exposes the functionality of asyncpg's executemany() and allows me to provide it a list of ValueObjects, but there is no fetch_many() method in QuartDB. Given the underlying functionality for this in asyncpg is there I was hoping I could get the fetch_many() method implemented. If you need me to create a PR to do so I can try, but I figured y'all could implement it faster than I could 😅.