Support for `WHERE` clause when usng `copy_to_table` method

Hi there!

Since version 12, PostgreSQL has supported a WHERE clause within COPY ... FROM statements:

COPY table_name [ ( column_name [, ...] ) ]
    FROM { 'filename' | PROGRAM 'command' | STDIN }
    [ [ WITH ] ( option [, ...] ) ]
    [ WHERE condition ]

It functions similarly to the WHERE clause used within SELECT statements, subject to some restrictions regarding subqueries.

Recently I've found myself in a situation where I am performing a bulk insert of CSV-formatted data from a remote machine, but would like to skip certain rows - such as those containing null values. From what I can tell, the existing Connection.copy_to_table helper doesn't support this, and I'd need to drop down to private API (the underlying Connection._protocol._copy_in machinery)

I'd appreciate some publicly exposed way of doing this - the simplest approach that comes to mind would be adding a where kwarg to Connection.copy_to_table as well as the Connection._format_copy_opts method. Usage could then look something along the lines of

await conn.copy_to_table("cool_table", source="important_file.csv", where="important_value = 42")

I'm not sure if this is satisfactory, though. There would also need to be some consideration regarding earlier Postgres versions that don't support COPY ... FROM using a WHERE clause.

Please let me know your thoughts - I'm happy to open a PR for implementation if necessary.

Empieza inspeccionando Connection.copy_to_table y Connection._format_copy_opts; después, sigue cómo llegan a la lógica privada Connection._protocol._copy_in. Comprueba cómo se gestiona la compatibilidad con las versiones de PostgreSQL e identifica las pruebas relevantes antes de decidir cómo debería comportarse la opción pública. Se considera completado cuando exista una forma compatible de aplicar una condición COPY ... FROM WHERE, con un comportamiento adecuado para versiones anteriores de PostgreSQL.