utils: chunked_vector: make begin()/end() const correct
utils: chunked_vector: make begin()/end() const correct

begin() of a const vector should return a const_iterator, to avoid
giving the caller the ability to mutate it.

This slipped through since iterator's constructor does a const_cast.

Noticed by code inspection.

avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla