Use operation name for generated operation name

[ruudk]

This allows writing things like:

private const string OPERATION = <<<'GRAPHQL'
    query ViewerProjects {
        viewer {
            login
            projects {
                name
                description
            }
        }
    }
    GRAPHQL;
public function __construct(
    #[GeneratedGraphQLClient(self::OPERATION)]
    public ViewerProjectsQuery $query,
) {}

As you can see, the class is now called ViewerProjectsQuery which looks a lot better than
ViewerProjects1d8480Query.