Fix/Implement various invite-related behaviors (#1023)
* Initial support for invite member count arg
* Fix IDiscordClient#GetInviteAsync behavior
- Previously, the GetInviteAsync method would return null since it couldn't be parsed as a simple RestInvite object. The object should be a RestInviteMetadata instead.
* Fix methods that didn't comply with the interface
* Change with_counts REST behaviour
* Remove unnecessary JSON prop
* Remove AcceptAsync