Make the transaction snapshot ID an ATTACH parameter

Hello !

I recently encountered a need where I wanted to `COPY` multiple tables within the same transaction snapshot ID.
As I understand it, duckdb already uses transaction snapshots to copy a single table in parallel.

The idea is to add another PostgreSQL specific option to the `ATTACH` command which would always use the given transaction snapshot when calling the function `PostgresScanConnect`.

This would allow duckdb to take consistent snapshots of Postgres databases, which I believe is quite useful.

What do you think about this feature ?

Thanks a lot for the amazing software !