Store native BSON types as-is in the MongoDB formatter

[ellotheth]

I like to throw MongoDB document IDs in my log context so I can cross-reference them later:

$log->info("{$user->email} logged in", [ 'actor' => $user->id ]);

Right now, \Monolog\Formatter\MongoDBFormatter flattens ObjectIds into arrays (e.g. [ 'oid' => 'object id string', 'class' => ObjectId::class ]). This PR allows all \MongoDB\BSON\Type implementations to pass through the formatter without additional processing.

[Seldaek]

Thanks