…ormational status codes (nicolas-grekas)

This PR was merged into the 4.3 branch.

Discussion
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[HttpClient] fix support for 103 Early Hints and other informational status codes

| Q             | A
| ------------- | ---
| Branch?       | 4.3
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

I learned quite recently how 1xx status codes work in HTTP 1.1 when I discovered the [103 Early Hint](https://evertpot.com/http/103-early-hints) status code from [RFC8297](https://tools.ietf.org/html/rfc8297)

This PR fixes support for them by adding a new `getInformationalStatus()` method on `ChunkInterface`. This means that you can now know about 1xx status code by using the `$client->stream()` method:

```php

$response = $client->request('GET', '...');

foreach ($client->stream($response) as $chunk) {
    [$code, $headers] = $chunk->getInformationalStatus();
    if (103 === $code) {
        // $headers['link'] contains the early hints defined in RFC8297
    }

    // ...
}
```

Commits
-------

34275bb [HttpClient] fix support for 103 Early Hints and other informational status codes