This PR was merged into the 4.3-dev branch.

Discussion
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[HttpClient] add MockHttpClient

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This PR introduces `MockHttpClient` and `MockResponse`, to be used for testing classes that need an HTTP client without making actual HTTP requests.

`MockHttpClient` is configured via its constructor: you provide it either with an iterable or a callable, and these will be used to provide responses as the consumer requests them.

Example:
```php
$responses = [
    new MockResponse($body1, $info1),
    new MockResponse($body2, $info2),
];

$client = new MockHttpClient($responses);
$response1 = $client->request(...); // created from $responses[0]
$response2 = $client->request(...); // created from $responses[1]
```

Or alternatively:
```php
$callback = function ($method, $url, $options) {
    return new MockResponse(...);
};

$client = new MockHttpClient($callback);
$response = $client->request(...); // calls $callback internal
```

The responses provided to the client don't have to be instances of `MockResponse` - any `ResponseInterface` works (e.g. `$this->getMockBuilder(ResponseInterface::class)->getMock()`).

Using `MockResponse` allows simulating chunked responses and timeouts:
```php
$body = function () {
    yield 'hello';
    yield ''; // the empty string is turned into a timeout so that they are easy to test
    yield 'world';
};
$mockResponse = new Mockresponse($body);
```

Last but not least, the implementation simulates the full lifecycle of a properly behaving `HttpClientInterface` contracts implementation: error handling, progress function, etc. This is "proved" by `MockHttpClientTest`, who implements and passes the reference test suite in `HttpClientTestCase`.

Commits
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8fd7584 [HttpClient] add MockHttpClient