Custom CursorIterator

还没有人认领这个 Issue。

I'd like to implement my own CursorIterator, but I can't find a straightforward way of getting it done via asyncpg public interface.
Currently it's created by CursorFactory, which in turn is created by PreparedStatement.cursor.
I can see two options here:

- Introduce an additional parameter of CursorFactory type in PreparedStatement.cursor which would be None by default and None means use standard CursorFactory
- do the same in PreparedStatement.__init__

跟踪 CursorFactory 以及 PreparedStatement.cursor 和 PreparedStatement.__init__ 入口点，以了解如何通过公共接口公开自定义 CursorIterator。确定哪个提议的 hook 符合现有 API，然后验证调用方可以提供自定义迭代器，同时默认行为保持不变。

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