Update offsetExists Function to Use Async Cache Check

[valzargaming]

Description

This PR updates the offsetExists function in the AbstractRepository class to enhance functionality by integrating an asynchronous cache check. The previous implementation used a direct parent call for offsetExists. The new implementation leverages the cache component, ensuring a more accurate check of the cache state.

Changes Made

• Updated offsetExists from: return parent::offsetExists($offset) to return await($this->cache->has($offset)).
• This change ensures the repository adheres to an asynchronous design pattern and integrates seamlessly with the cache system.

Benefits

• Adds forward compatibility for the previously deprecated offsetExists function, allowing it to remain functional with the new cache system.

[bwoebi]

I'm randomly seeing this - and would like to note that this will be quite the breaking change as it would require users to put the offsetExists call into an async callback. At least unless you make all discord event callbacks async too.
Currently awaiting (without wrapping into async) will ultimately stall ratchet in its callbacks, causing the current pending websocket packets to be dequeued after the callback possibly causing out-of-order callback delivery. (and this may cause the connection to become invalid for example)

[valzargaming]

I'm more than a little disappointed that you're the only person to point that out.