Want v8 support in serenity-voice-model

[FelixMcFelix]

Voice gateway version 8 includes several breaking changes to message formats:

• All received (server-to-client) messages will include a seq integer value.
• Heartbeat payload are changing from int -> struct { t: int } on the wire.
• Resume requests will include a seq_ack field, corresponding to the last seen seq at a client.

Making these changes is not pressing -- we're explicitly advertising v4 gateway support, so we do have some mileage as that pathway is not being deprecated as yet. At most this enables extra functionality around gateway message replay -- see serenity-rs/songbird#247.