Add support for server side interceptors

[srikrsna-buf]

Add support for server side interceptors. Reuses the same types of the client side interceptors.

Example:

import { fastify } from "fastify";
import routes from "./connect";
import { fastifyConnectPlugin } from "@connectrpc/connect-fastify";

const server = fastify();

await server.register(fastifyConnectPlugin, {
 routes,
  interceptors: [
    (next) => async (req) => {
      console.log("Request:", req);
      return await next(req);
    },
  ],
});

await server.listen({
  host: "localhost",
  port: 8080,
});

Closes #527