Allow webhooks to opt into automod

[muddyfish]

For the POST and PATCH methods for sending/editing webhooks, it would be useful to be able to opt-in to automod rule filtering.

Right now, there's no good way to cache rules on startup (GUILD_CREATE does not include them), and even if there was, we cannot support the built in rules set (spam/common flagged words).

For those two methods, I'd want a boolean field for opt-in to automod, with the message being rejected in case it's blocked.