Restore ApolloServerStandalone functionality

[glasser]

The batteries-included apollo-server package has been deleted from the version-4 branch. Its functionality should be restored as a new class with a different name in the main package, like ApolloServerStandalone or something.

[glasser]

Some of the cors tests deleted in 81c7f7b will probably be useful for the standalone server.

[glasser]

@trevor-scheer and I ran into some issues related to generic types and TContext trying to make this a class. My current idea is to make this a function instead (pretty similar to what @abernix originally proposed in #3184 though with a bit more async-ness). This noun-named httpServer is analogous to expressMiddleware.

// or maybe you have to deep-import httpServer to avoid importing `express` otherwise?
// or maybe the `express` import is a runtime import() instead.
import { ApolloServer, httpServer } from '@apollo/server';
const server = new ApolloServer<MyContext>({ typeDefs, resolvers });
await httpServer(server, {
  context: async ({ req, res }) => new MyContext(req),
}).listen(1234);

where httpServer is a function that synchronously returns something like

class ApolloHttpServer {
  async listen(...args): Promise<{url: string}> {
    await this.server.start();
    await promisified-server-listen
  }
}

[glasser]

@trevor-scheer done?

[trevor-scheer]

Closed via #6420