Add bson_iter_init_from_data function

The mongocxx driver would benefit from the ability to directly initialize a bson_iter_t from a data buffer, rather than having to repetitively call bson_init_static merely to call bson_iter_init. As bson_iter_init only uses the passed bson_t to extract a buffer pointer and length, we'd like an alternative initializer for bson_iter_t that takes those directly.

A PR will follow shortly.