feat($resource): add support for `request` and `requestError` interceptors (#15674)

This commit adds `request` and `requestError` interceptors for `$resource`, as
per the documentation found for `$http` interceptors. It is important to note
that returning an error at this stage of the request - before the call to
`$http` - will completely bypass any global interceptors and/or recovery
handlers, as those are added to a separate context. This is intentional;
intercepting a request before it is passed to `$http` indicates that the
resource itself has made a decision, and that it accepts the responsibility for
recovery.

Closes #5146

BREAKING CHANGE:

Previously, calling a `$resource` method would synchronously call
`$http`. Now, it will be called asynchronously (regardless if a
`request`/`requestError` interceptor has been defined.

This is not expected to affect applications at runtime, since the
overall operation is asynchronous already, but may affect assertions in
tests. For example, if you want to assert that `$http` has been called
with specific arguments as a result of a `$resource` call, you now need
to run a `$digest` first, to ensure the (possibly empty) request
interceptor promise has been resolved.

Before:
```js
it('...', function() {
  $httpBackend.expectGET('/api/things').respond(...);
  var Things = $resource('/api/things');
  Things.query();

  expect($http).toHaveBeenCalledWith(...);
});
```

After:
```js
it('...', function() {
  $httpBackend.expectGET('/api/things').respond(...);
  var Things = $resource('/api/things');
  Things.query();
  $rootScope.$digest();

  expect($http).toHaveBeenCalledWith(...);
});
```