PHP-659: Add method prototypes for GridFS classes

This replaces #281. I addressed most of the comments in the previous PR, except:

 * https://github.com/mongodb/mongo-php-driver/pull/281#discussion_r2750919 - Was it important to have `$this->chunks` verbatim? I though the proto text was clear that we are dropping both collections.
 * https://github.com/mongodb/mongo-php-driver/pull/281#discussion_r2751160 - `_id` can be anything, afaik.

While implementing other feedback items, I opened the following PHP tickets:

 * https://jira.mongodb.org/browse/PHP-726
 * https://jira.mongodb.org/browse/PHP-727