Update versioned API getters (#765)

* Make mongoc_optional_t method args const

* Add const modifier to mongoc_server_api_t getters

* Fix wrong name for mongoc_server_api_t getter

* Add getter to retrieve version from mongoc_server_api_t

* Add documentation for mongoc_server_api_t getters

* Add documentation for mongoc_optional_t

* Fix CR feedback