normalize payloads in agent message

[xtuc]

{"agentid":"b17ac604-b7af-46b1-a7a7-c3c25405ef6f","type":"Mutation","payload":[{"method":"steer","arguments":[0,1]}]}
vs this shit:
{"agentid":"b17ac604-b7af-46b1-a7a7-c3c25405ef6f","type":"Mutation","payload": {mutations:[{"method":"steer","arguments":[0,1]}]}

[xtuc]

https://github.com/ByteArena/bytearena/blob/master/arenaserver/communication.go#L205