libnl++: add new func to send raw data with len

Some netlink attrib does not ending with null char.
Currently req.add string will always sending ending null character.
Add a new func called addBuffer that does not send ending null
character.

Bug: 238794143
Test: build, manual test
Change-Id: If51380cab95b2ae725673301b429b9e9889c5b0a