JSON deserialization DoS

[samczsun]

If this is a concern, this needs to switch to using a decoder too

https://github.com/flashbots/mev-boost/blob/c03c1674a19346ff6ff27c6c562f15b6d8d33f8a/server/utils.go#L71

Also, a lot of types will deserialize a raw []bytes which has limitless potential to DoS (although it will then result in an invalid message, so hey).