Please give ConnectionFactory an interface

### Feature Request

#### What

With your version 3 you made ConnectionFactory final, forcing us to wrap the service instead of extending it. This works fine, but there is no interface we could be using.

#### Why

We use a custom ConnectionFactory to inject additional services into our custom connection that we need.

Also we register some additional and complex database types in there that are not compatible with ConnectionFactory::initializeTypes.

#### How

Implementing an interface like this would already suffice:

```php

interface ConnectionFactoryInterface {
public function createConnection(array $params, ?Configuration $config = null, array $mappingTypes = []): Connection;
}

```

Thank you

Start by locating the existing ConnectionFactory entry point and its tests, then trace how the bundle obtains and uses the factory. Define the interface around the createConnection signature shown in the issue and verify that custom implementations can be supplied without changing existing behavior; add or update focused tests for both the default factory and injection path.

Written by the indexing model from the issue text.