Implement Generator-based response streaming

[ivanpepelko]

POC of proper Response streaming.

I'm currently working on an app which uses Response streaming. It's a simple use case, where progress updates are pushed to the client. The team uses both symfony-cli runtime and Roadrunner.

With this PR, I'd like to open a discussion on whether this is the correct approach (eg. I'm not sure this extended version of StreamedResponse fits into this project).

IMPORTANT: This will not work without appropriate change in symfony/http-kernel, which wraps Response callback and does not return the value (which must be a Generator instance) (PR).

This update changes the client API. Before:

use Symfony\Component\HttpFoundation\StreamedResponse;

$response = new StreamedResponse();
$response->setCallback(function (): void {
    echo 'Hello World';
    flush();
    sleep(2);
    echo 'Hello World';
    flush();
});

After:

use Baldinof\RoadRunnerBundle\Http\StreamedResponse;

$response = new StreamedResponse();
$response->setCallback(function (): void {
    yield 'Hello World';
    sleep(2);
    yield 'Hello World';
});

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At the moment, in my project I've added composer hook to patch files from the PR. This is not an optimal solution as it adds unnecessary maintenance overhead.