SVS dynamic indices' add_points()/delete_points() to return number of actually added/deleted vectors.

Dynamic index modification methods behavior should be changed as follow:
* `add_points()` should allow to override existing vectors (vectors with same labels) and return the number of new vectors only.
* `delete_points()` should accept non-existing external ids and return the number of vectors which are actually deleted.

https://github.com/intel/ScalableVectorSearch/blob/ec4260c8764792ddd89982f324362d011ae95150/bindings/c/src/index.hpp#L135-L151