Add support for gridfs to the new C driver.

Basic spec:

• Add a new mongoc_gridfs_t structure as a central api point.
• Fetch a mongoc_gridfs_t from a mongoc_client_t with something like mongoc_client_get_gridfs(...) similar to how we get a database or collection.
• Create a mongoc_stream_t sub-structure for working with a file in gridfs so that we can support streaming files to and from the MongoDB server.