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

还没有人认领这个 Issue。

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
```py
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.

首先检查 Connection.copy_to_table 和 Connection._format_copy_opts，然后跟踪它们如何进入私有的 Connection._protocol._copy_in 机制。检查如何处理 PostgreSQL 版本兼容性，并在决定 public option 应如何行为之前，找出相关测试。完成的标准是：提供一种受支持的方式来应用 COPY ... FROM WHERE 条件，并对较早的 PostgreSQL 版本采取适当的行为。

由索引模型根据 Issue 内容生成。