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.

Bắt đầu bằng cách kiểm tra Connection.copy_to_table và Connection._format_copy_opts, sau đó lần theo cách chúng đi tới cơ chế Connection._protocol._copy_in private. Kiểm tra cách xử lý khả năng tương thích với các phiên bản PostgreSQL và xác định các bài kiểm thử liên quan trước khi quyết định tùy chọn public nên hoạt động như thế nào. Được xem là hoàn thành khi có một cách được hỗ trợ để áp dụng điều kiện COPY ... FROM WHERE, với hành vi phù hợp cho các phiên bản PostgreSQL cũ hơn.