Implement async query functionality (MySQL and PostgreSQL)

[nacholibre]

Q	A
Type	feature

Summary

Quite often I find myself needing async queries in doctrine/orm. In order to implement them first dbal needs to have that functionality.

This PR is me prototyping (consider it proof of concept) for async functionality. Just to note I've used some AI tools to help me prototype and understand the codebase.

I don't consider this as finished product, I just want to get some feedback from the maintainers if my approach is in the right direction and general thoughts about the need of such feature.

The API is as follows:

// Create queries using QueryBuilder
$qb1 = $conn->createQueryBuilder()
    ->select('*')
    ->from('users')
    ->where('status = :status')
    ->setParameter('status', 'active');

$qb2 = $conn->createQueryBuilder()
    ->select('COUNT(*)')
    ->from('orders')
    ->where('created_at > :date')
    ->setParameter('date', '2024-01-01');

// Execute in parallel using the helper method
$results = $conn->executeQueriesAsync([
    AsyncQuery::fromQueryBuilder($qb1),
    AsyncQuery::fromQueryBuilder($qb2),
]);

$users = $results[0]->fetchAllAssociative();
$orderCount = $results[1]->fetchOne();

It works without the QueryBuilder as well

// Execute multiple slow queries in parallel
$results = $connection->executeQueriesAsync([
    new AsyncQuery('SELECT * FROM users WHERE status = $1', ['active']),
    new AsyncQuery('SELECT COUNT(*) as total FROM orders'),
    new AsyncQuery('SELECT AVG(price) as avg_price FROM products WHERE category_id = $1', [5]),
]);

Thanks for your time.

[derrabus]

Hello and thank you for your pull request. I haven't found the time to review it yet, just one thing: We don't accept new features on the 3.10.x branch anymore. You will need to target our next feature release which currently is 4.5.x.