support max_body_size in child spec options

[aseigo]

With the new max_body_size support, body sizes can be controlled .. but the max_body_size parameter is not accepted as an option to the child spec of an endpoint.

We currently have this starting in a supervision tree:

children = [
      GrpcReflection,
      {
        GRPC.Server.Supervisor,
        [
          endpoint: Ujs.Router.Endpoint,
          port: port,
          start_server: true,
          max_body_size: Application.get_env(:ujs, :grpc_server_max_body_size)
        ]
      }
    ]

This currently fails due to the missing max_body_size key in the Keyword.validate call. This fixes that issue, and the above supervision tree starts and the max body size is correctly defined.