Expose RCTNetworking as a public 'Networking' API (#25718)

Summary:
This PR introduces the `EventSource` web standard as a first-class networking feature in React Native. In the discussion we had in February at react-native-community/discussions-and-proposals#99, cpojer indicated that the RN maintainers would be willing to accept a PR to offer this functionality.

The linked discussion goes into detail about why this change must happen in React Native Core as opposed to a community library, but the tl;dr is that `XmlHttpRequest` doesn't let you do streaming in a resource-efficient way, since it holds onto the entire response buffer until the request is complete. When processing a stream that might last for a long time, that's not ideal since there might be a lot of data in that buffer that is now useless to maintain.

For more information about EventSource and server-sent events, check out these links:
* [EventSource on MDN](https://developer.mozilla.org/en-US/docs/Web/API/EventSource)
* [Using server-sent events on MDN](https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events)
* [WHATWG spec for server-sent events](https://html.spec.whatwg.org/multipage/server-sent-events.html)

I've tried as best as I can to satisfy the linked specification so that this is as standard as possible.

One of the projects I maintain has an ideal use case for this feature. The SDK for MongoDB Stitch (a backend-as-a-service for the MongoDB database) has the ability to open a "change stream" to watch for changes that happen on a database. However, in our JavaScript SDK, this feature depends on `EventSource`, because the backend service implements the one-way streaming protocol with server-sent events. We know there is demand for this feature because users have requested it: mongodb/stitch-js-sdk#209.

If this PR will be accepted, I am happy to update the `Networking` documentation at https://facebook.github.io/react-native/docs/network

## Changelog

[JavaScript] [Added] Implements the `EventSource` web standard in `Libraries/Networking`
[JavaScript] [Added] Exposes the `EventSource` implementation in `Libraries/Core/setUpXHR.js`
Pull Request resolved: #25718

Test Plan:
To test the `EventSource` implementation, I added a comprehensive set of unit tests that cover the basic functionality, as well as edge cases that are laid out in the spec. See `EventSource-test.js` for the cases that the tests handles. For convenience, I've also included the test descriptions as produced by the `jest` test output here.

```
 PASS  Libraries/Network/__tests__/EventSource-test.js
  EventSource
    ✓ should pass along the correct request parameters (527ms)
    ✓ should transition readyState correctly for successful requests (4ms)
    ✓ should call onerror function when server responds with an HTTP error (2ms)
    ✓ should call onerror on non event-stream responses (1ms)
    ✓ should call onerror function when request times out (1ms)
    ✓ should call onerror if connection cannot be established (1ms)
    ✓ should call onopen function when stream is opened (1ms)
    ✓ should follow HTTP redirects (2ms)
    ✓ should call onmessage when receiving an unnamed event (2ms)
    ✓ should handle events with multiple lines of data (1ms)
    ✓ should call appropriate handler when receiving a named event (1ms)
    ✓ should receive multiple events (1ms)
    ✓ should handle messages sent in separate chunks (1ms)
    ✓ should forward server-sent errors
    ✓ should ignore comment lines (1ms)
    ✓ should properly set lastEventId based on server message (1ms)
    ✓ should properly set reconnect interval based on server message
    ✓ should handle messages with non-ASCII characters (1ms)
    ✓ should properly pass along withCredentials option (3ms)
    ✓ should properly pass along extra headers (1ms)
    ✓ should properly pass along configured lastEventId (2ms)
    ✓ should reconnect gracefully and properly pass lastEventId (9ms)
    ✓ should stop attempting to reconnect after five failed attempts (2ms)
```

As a manual E2E test, I also added streaming support to the Stitch React Native SDK, and tested it with my React Native EventSource implementation, and confirmed that our `EventSource`-based streaming implementation worked with this `EventSource` implementation.
* Source code for E2E app test: https://gist.github.com/adamchel/6db456c1a851ed7dd20b54f6db3a6759
* PR for streaming support on our React Native SDK: mongodb/stitch-js-sdk#294
* Very brief video demonstrating E2E functionality: https://youtu.be/-OoIpkAxmcw

Differential Revision: D17283890

Pulled By: cpojer

fbshipit-source-id: 0e9e079bdb2d795dd0b6fa8a9a9fa1e840245a51

Libraries/react-native/react-native-implementation.js

@@ -68,6 +68,7 @@ import typeof LayoutAnimation from '../LayoutAnimation/LayoutAnimation';
 import typeof Linking from '../Linking/Linking';
 import typeof NativeDialogManagerAndroid from '../NativeModules/specs/NativeDialogManagerAndroid';
 import typeof NativeEventEmitter from '../EventEmitter/NativeEventEmitter';
+import typeof Networking from '../Network/RCTNetworking';
 import typeof PanResponder from '../Interaction/PanResponder';
 import typeof PermissionsAndroid from '../PermissionsAndroid/PermissionsAndroid';
 import typeof PixelRatio from '../Utilities/PixelRatio';
@@ -330,6 +331,9 @@ module.exports = {
   get NativeEventEmitter(): NativeEventEmitter {
     return require('../EventEmitter/NativeEventEmitter');
   },
+  get Networking(): Networking {
+    return require('../Network/RCTNetworking');
+  },
   get PanResponder(): PanResponder {
     return require('../Interaction/PanResponder');
   },