add function to send stickers

[kbuika]

Hey @DaggieBlanqx, I completed the function that sends stickers to individual recipients. However, I did not get the chance to test it since I have issues with my access_token. If this is a potential issue, feel free to mark this PR as pending work in progress.

I also caught a possible error on line 457, the function throws an error when the file_path and url are provided.

Also, I need some clarification, on the pending functions to get user information, does that mean getting the recipient's information? or the sender's information?

Finally, it would make my life easier if you granted me access to the testing environment you are using or rather, guide me on how to set up a similar environment, this way, we'll be working with the same standard.