Implement encryption of file

I use object composition, so I call mongoc_gridfs_file_readv inside mongoc_gridfs_cnv_file_readv, I do the same for all functions even if there is no additional logic. file->flags are set on object creation and I just check If we try to decrypt without set aes key - it's reading error. If read() returns 0 bytes it means we read all data and we should check data integrity using EAX tag.