PHPC-2447: Extend Iterator interface in CustorInterface

[alcaeus]

PHPC-2447

Making this change now will make typing changes in PHPLIB-954 much easier.

[GromNaN]

Noted that we already have a test that checks that CursorInterface implements Iterator.

[GromNaN]

We need to specify that CursorInterface::key() return ?int instead of mixed.

mongo-php-driver/src/MongoDB/Cursor.stub.php

Line 24 in 525dadb

public function key(): ?int {}

This is necessary if we want CodecCursor to accept CursorInterface instead of Cursor.

[alcaeus]

Ah, good point. I'll update the types in #1658.