PHPC-2478: Implement API for bulkWrite command

[jmikola]

https://jira.mongodb.org/browse/PHPC-2478

[GromNaN]

By placing the execute method on this side, it becomes essential to be able to mock this class. But this class is final.

How do you test a function that creates a BulkWriteCommand and executes it?

function saveDocuments(Manager $manager, array $documents) {
    $command = new BulkWriteCommand();
    foreach ($documents as $document) {
        $command->replaceOne(/** do something very domain specific */);
    }
    // I have no idea how to track the call to $manager
    $command->execute($manager);

    // This implementation would be easier to mock
    $manager->execute($command);
}

[jmikola]

Thanks for the feedback here. I clearly don't deal with mocking objects anymore, so I overlooked how this would hinder that. I suppose that takes us back to Manager::executeBulkWriteCommand(), and a corresponding Server method.

[jmikola]

Revisiting this since we discussed it earlier in the week with @alcaeus. In terms of mocking, having execute() here isn't much different than the Manager and Server classes, which are also both final.

We also discussed having execute() methods oh other objects, and I believe only Command would be problematic since that currently relies on several variations (read, read/write, write) and to replace that with a singular execute() method would require PHPC to inspect command documents.

Anyway, I'm going to proceed with BulkWriteCommand::execute() for now and we can revisit later if needed.