limit headers size

[ltitanb]

The PBS server is assumed to be trusted and accessible only by the beacon node but we could still add a limit on the headers size

mev boost for reference
https://github.com/flashbots/mev-boost/blob/develop/server/service.go#L187

[0xvv]

Looking into this but I think axum does not give access to hyper's configuration for the server so this would require rewriting the PbsService.run() without axum
I'm trying to find a light refactoring to accomplish this as this does not necessarily seem worth a big code change

[ltitanb]

agreed, definitely not worth massive changes considering that the beacon node is assumed to be trusted by the sidecar