libnl++: fix std::string_view handling

This is a partial revert of If51380cab95b2ae725673301b429b9e9889c5b0a

Changes:
- combine add() and addBuffer - how both skip the trailing '\0'
- assert that add<A>() is only used for POD
- implement add overload for std::string_view

Bug: 372814636
Test: m
Change-Id: I08d8825636a3a908e0ebd3bd312580a27505c330