Add the HttpInterceptor directly into the core library

[Badisi]

Currently there is an HttpInterceptor in the Angular implementation, which allow the access token to be automatically added to any request header. This feature is made possible thanks to '@angular/common/http' but could easily be rewritten in pure javascript to be made available directly in the core library.

---

Some snippets examples:

Fetch

const {fetch: origFetch} = window;
window.fetch = async (...args) => {
  console.log("fetch called with args:", args);
  const response = await origFetch(...args);
  
  /* work with the cloned response in a separate promise
     chain -- could use the same chain with `await`. */
  response
    .clone()
    .json()
    .then(body => console.log("intercepted response:", body))
    .catch(err => console.error(err))
  ;
    
  /* the original response can be resolved unmodified: */
  //return response;
  
  /* or mock the response: */
  return {
    ok: true,
    status: 200,
    json: async () => ({
      userId: 1,
      id: 1,
      title: "Mocked!!",
      completed: false
    })
  };
};

// test it out with a typical fetch call
fetch("https://jsonplaceholder.typicode.com/todos/1")
  .then(response => response.json())
  .then(json => console.log("original caller received:", json))
  .catch(err => console.error(err))
;

XMLHttpRequest

(function(open) {
    XMLHttpRequest.prototype.open = function(XMLHttpRequest) {
      var self = this;
      this.addEventListener("readystatechange", function() {
        if (this.responseText.length > 0 && 
            this.readyState == 4 && 
            this.responseURL.indexOf('www.google.com') >= 0) {
  
          Object.defineProperty(self, 'response', {
            get: function() { return bValue; },
            set: function(newValue) { bValue = newValue; },
            enumerable: true,
            configurable: true
          });
          self.response = 'updated value' // Intercepted Value 
        }
      }, false);
      open.apply(this, arguments);
    };
})(XMLHttpRequest.prototype.open);