Improve Fixture and FixtureRollback interface

[Fabian Schmengler />]

It's currently a bit cumbersome to use the Fixture and FixtureRollback classes together with the Builder classes.

I would like to make it easier to create a bunch of fixtures and roll them back at once, e.g. with a fixture pool:

class SomeTest
{
  private FixturePool $fixtures;
  protected function setUp()
  {
    $this->fixtures = new FixturePool();
    $this->fixtures->addCustomer(CustomerBuilder::aCustomer()->build(), 'customer1');
    $this->fixtures->addCustomer(CustomerBuilder::aCustomer()->build(), 'customer2');
    $this->fixtures->addProduct(ProductBuilder::aSimpleProduct()->build(), 'product1');
  }
  protected function tearDown()
  {
    $this->fixtures->rollBackAll();
  }

  public function testSomething()
  {
    $customer1 = $this->fixtures->getCustomer('customer1'); // returns CustomerFixture object
    $product = $this->fixtures->getProduct(); // key is optional, returns first element by default
    // ....
  }
}