Add reason to GUILD_BAN_ADD event

[nicosammito]

Currently the reason for a ban isn't included in the GUILD_BAN_ADD event.
So to get the specific reason you need to make a request to the GET Ban route.
It would be way better to include this also in the GUILD_BAN_ADD event to get the reason directly instead of making another request to Discord REST API.

The new payload could be:

FIELD	TYPE	DESCRIPTION
guild_id	snowflake	id of the guild
user	a user object	the banned user
reason	?string	the reason for the ban