Add negotiate

[zackliu]

Note: the engine.io.js file is the generated output of make engine.io.js, and should not be manually modified.

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

The engine.io connection directly connects to the hostname, path.

New behaviour

If negotiatePath is set, engine.io will make a fetch to the <hostname>/<negotiatePath> to get a JSON encoded response first.

{
  "url", "http://hostname/path",
  "token": "<token>"
}

if succeeded, the engine.io will parse the url got in negotiation and replace it with the previous hostname, path, port and protocol in opts.
As the opts has changed, everything is unchanged then (e.g when constructing a transport, it still read from opts).

Another change is adding a token in opts. If token is set, it will be used in Authorization for long polling, and access_token query for websocket. This token can be set in opts directly but it can also be override in negotiation.

Other information (e.g. related issues)

This is the implementation of issue socketio/socket.io#4825

[zackliu]

@darrachequesne We can discuss whether the design is acceptable, and I will write some more test.

[zackliu]

@darrachequesne Do you have any comments on the design?

[darrachequesne]

Hi @zackliu,

Thanks for your work on this.

However, I still don't think the use case is common enough for this to be included here in the package.

[zackliu]

@darrachequesne I think there're two parts in it.

1. Added a token in opts, which can be used in Authorization in long polling and access_token query in websocket
2. Add a negotiate opts and replace the correlating fields in opts.

I admit we can use a separate library for the second purpose but what about the first one? Do you think it's acceptable to have such a property as it's hard to achieve outside this package?

[darrachequesne]

For 1, the transportOptions option should be sufficient I think:

import { io } from "socket.io-client";

const socket = io({
  transportOptions: {
    polling: {
      extraHeaders: {
        Authorization: myToken
      }
    },
    websocket: {
      query: {
        access_token: myToken
      }
    }
  }
});

Reference: https://socket.io/docs/v4/client-options/#transportoptions