WebSocket API change to make room for other connection options (SSL pinning)

Summary:
This is a simple groundwork PR to allow options to be passed to the `WebSocket` constructor. It represents a minor change to an undocumented part of the API, moving `headers` to within `options`.

This will be a BC for anyone manually specifying headers other than `origin` but a) that's not a common use case with WebSockets and b) it's not documented even in code and wouldn't currently pass a flow check.

NB: The third argument to the WebSocket constructor isn't part of the W3C spec, so I think this is a good place for RN-specific named parameters, better than adding a fourth argument. `protocols` needs to stay where it is, in line with the spec.

If this goes through I'd like to build on it by adding an additional connection option for SSL certificate pinning, as already supported by the underlying `okhttp` and `RCTSRWebSocket`. It could later be expanded for various other uses.

Currently, there's no way for a `WebSocket` user to specify any connection options other than url, protocol and headers. The fact that `WebSocket` connects in its constructor means any options have to go in there.

Connect to a websocket server using iOS and Android, observe the connection headers:
1. Without specifying `origin`, the default header should be set
2. Specifying it in the old way `new WebSocket(url, protocols, { origin: 'customorigin.com' })`
3. Specifying it in the new way `new WebSocket(url, protocols, { headers: { origin: 'customorigin.com' }})`.

I've tested myself using the test app with iOS and Android.
Closes https://github.com/facebook/react-native/pull/15334

Differential Revision: D5601675

Pulled By: javache

fbshipit-source-id: 5959d03a3e1d269b2c6775f3e0cf071ff08617bf