Add requestGate handler option

[timostamm]

This PR adds a new handler option:

  /**
   * An optional gate that runs after request headers are available, but before
   * any request message is received, decompressed, or parsed.
   *
   * This is the right place to reject unauthenticated requests cheaply. Throw
   * a ConnectError to end any RPC without reading the body.
   *
   * The gate receives the HandlerContext for the call. It may set context
   * values for interceptors and the implementation to consume, and set
   * response headers or trailers.
   */
  requestGate?: (context: HandlerContext) => void | Promise<void>;

To authenticate requests on a server, prefer this option over interceptors. For unary RPCs, interceptors run after the request message is received, decompressed, and parsed. The request gate turns away requests without spending anything on them.

import * as http from "http";
import routes from "./connect";
import { connectNodeAdapter } from "@connectrpc/connect-node";
import { Code, ConnectError } from "@connectrpc/connect";
import type { HandlerContext } from "@connectrpc/connect";
// This can come from an auth library like passport.js
import { authenticate } from "./authenticate";

function authGate(context: HandlerContext) {
  if (authenticate(context.requestHeader.get("Authorization")) === undefined) {
    throw new ConnectError("User not authenticated", Code.Unauthenticated);
  }
}

http
  .createServer(
    connectNodeAdapter({
      routes,
      requestGate: authGate,
    }),
  )
  .listen(8080);