This PR was merged into the 4.3-dev branch.

Discussion
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[HttpClient] Add a ScopingHttpClient

| 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 is a follow up of #30592 by @XuruDragon, with two main differences:
- I think `ScopingHttpClient` might be a better name for what is called a `ConditionalHttpClient` there,
- the `FrameworkBundle` part is removed so that it can be submitted separately later on.

With a `ScopingHttpClient`, you can add some default options conditionally based on the requested URL and a regexp that it should match. This allows building clients that add e.g. credentials based on the requested scheme/host/path.

When the requested URL is a relative one, a default index can be provided - whose corresponding default options (the `base_uri` one especially) will be used to turn it into an absolute URL.

Regexps are anchored on their left side.

E.g. this defines a client that will send some github token when a request is made to the corresponding API, and will not send those credentials if any other host is requested, while also turning relative URLs to github ones:
```php
$client = HttpClient::create();
$githubClient = new ScopingClient($client, [
    'http://api\.github\.com/' => [
        'base_uri' => 'http://api.github.com/',
        'headers' => ['Authorization: token '.$githubToken],
    ],
], 'http://api\.github\.com/');
```

Of course, it's possible to define several regexps as keys so that one can create a client that is authenticated against several hosts/paths.

Commits
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1ee0a11 [HttpClient] Add a ScopingHttpClient