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

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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.

Beginne mit der Untersuchung von Connection.copy_to_table und Connection._format_copy_opts und verfolge anschließend, wie sie zur privaten Connection._protocol._copy_in-Logik gelangen. Prüfe, wie die Kompatibilität mit PostgreSQL-Versionen behandelt wird, und ermittle die relevanten Tests, bevor du entscheidest, wie sich die öffentliche Option verhalten soll. Als erledigt gilt die Aufgabe, wenn es eine unterstützte Möglichkeit gibt, eine COPY ... FROM WHERE-Bedingung anzuwenden, mit angemessenem Verhalten für frühere PostgreSQL-Versionen.

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