Allow access to calculated query complexity

[ruudk]

Early return when complexity is disabled
This allows other validation rules to depend on the QueryComplexity rule, and disable the query complexity completely on some condition.

$rules[QueryComplexity::class] = $queryComplexity = new QueryComplexity($this->maxQueryComplexity);

// Always enable the query complexity rule, but ignore for introspection queries
$rules[WhenIntrospection::class] = new WhenIntrospection(function () use ($queryComplexity) : void {
    $queryComplexity->setMaxQueryComplexity(QueryComplexity::DISABLED);
});

Allow access to calculated query complexity
This allows you to read the calculated query complexity. A use case could be: add a rule that logs when queries are detected with a complexity higher than X.

This helps with #1204

$rules[QueryComplexity::class] = $queryComplexity = new QueryComplexity($this->maxQueryComplexity);

$rules[WhenLeaveOperation::class] = new WhenLeaveOperation(function () use ($queryComplexity) : void {
    if ($queryComplexity->getQueryComplexity() >= 400) {
        $this->logger->info('Found a query with high complexity');
    }
});

[ruudk]

@spawnia this one is ready for review, whenever you have time.