CreateDowngradingHandler takes a gRPC server and a plain HTTP handler, and returns an HTTP handler that has the capability of handling HTTP requests and gRPC requests that may require downgrading the response to gRPC-Web or gRPC-WebSocket.

PreferGRPCWeb instructs the server to always send a downgraded gRPC-Web response if the client indicates it accepts gRPC-Web responses.

Option is an object that controls the behavior of the downgrading gRPC server.