Add recurrence_rule to GuildScheduledEventParams

[jannes-io]

Currently we can't get/set recurrence on scheduled events.

Ref for GuildScheduledEvent(Params): https://discord.com/developers/docs/resources/guild-scheduled-event#create-guild-scheduled-event
Ref for recurrence_rule: https://discord.com/developers/docs/resources/guild-scheduled-event#guild-scheduled-event-recurrence-rule-object

[jannes-io]

I made an attempt and it seems to be just adding some more structs, but it's a little more complex with the frequency rules. I'm using this in a project to further my Go skills so I'm not comfortable contributing to a library yet.

Could anyone take a look at this? I'm even willing to put a small bounty on this issue if that would get it any more attention.. I'd really like to continue working in Go, but I'll have to switch to either the native Discord REST API, or another language like JS (discordjs) if I can't apply recurrence rules to events, it's a requirement for my project.