Add the experimental webhook support

[colbster937]

What feature do you want to see added?

I think that the experimental webhook format looks much better than having the bot send the messages, and is the only reason why i dont use this plugin on my network. Also make the join and leave messages look like they do in discordsrv with the player's face on the embed:

also make sure to add a way in the config to change the way that the player head is retrieved, for example https://minotar.net/helm/. other than that this is an amazing plugin and it fully supports waterfall 1.21 even though it doesnt say that. amazing work!