Allow binding `websocketPort` to `port`

[cnuss]

Description

In the following serverless.yml configuration:

custom:
  serverless-offline:
    port: 3000
    websocketPort: 3000

The following error occurs at startup:

✖ Unexpected error while starting serverless-offline server on port 3000: { Error: listen EADDRINUSE: address already in use ::1:3000
      at Server.setupListenHandle [as _listen2] (node:net:1898:16)
      at listenInCluster (node:net:1946:12)
      at GetAddrInfoReqWrap.doListen [as callback] (node:net:2116:7)
      at GetAddrInfoReqWrap.onlookup [as oncomplete] (node:dns:111:8)
    code: 'EADDRINUSE',
    errno: -98,
    syscall: 'listen',
    address: '::1',
    port: 3000 }

Motivation and Context

In a lot of HTTP + Websocket services, the Websocket and HTTP server can share the same port since they are on different protocols.

This PR:

• Introduces a AbstractHttpServer superclass which sets up commonalities between the various HttpServer classes most likely due to copy-paste of them over time
• Has helper functions so the HttpServer classes are more uniform
• Uses the Lambda class to maintain a map of which HttpServer is on which port
• Uses the Map() to have a single HttpServer created once and is the sole source of truth for all HttpServer instances
• Updated HttpServer in the websocket class override superclass functions to bind on the correct server.

Finally:

• In the event the websocketPort is set to the same value as port, the @connections API is hosted on the HttpServer created by the Lambda class. Less is more, right?!

How Has This Been Tested?

• All existing tests work after the refactor
• The websocket-oneway and websocket-twoway integration tests were copied with the new shared port configuration and also work.

[cnuss]

hi @dherault and @DorianMazur could you take a look at this PR? Thanks!