Remove custom AsyncIterators subclasses to use Generators

[DA-344]

Summary

Remove AsyncIterators subclasses accross the lib to start using yield item.

What is the feature request for?

The core library

The Problem

Currently, the library has to mantain a bunch of custom AsyncIterator subclasses used for paginated endpoints, such as Messageable.pins, Guild.bans, etc.

The Ideal Solution

This should change to be typed as (typing or collections.abc).AsyncIterator[YieldType], be converted into async def functions, and simply yield the items.

This would also remove everything related to .flatten so it is better used as l = [i async for i in asynciterator].

An example from Guild.bans:

async def bans(self, params) -> AsyncIterator[Ban]:
    # process stuff

    for result in ...:  # converted mapping of bans for example
        yield result

The Current Solution

No response

Additional Context

No response