Provide access to _id when creating GridFS file

A newly created mongoc_gridfs_file_t apparently doesn't have access to its chunks, so I need to refetch the file to return a "complete" mongoc_gridfs_file_t. This is the same behavior as with the previous C driver, only that the previous driver gave you access to the ObjectId which would allow you to refetch the file immediately. Fetching by name is obviously ambiguous.

So please either add a function that gives you access to a GridFS file's ObjectId, or assign the chunks to the file immediately after uploading them.