zebra,lib: command to only install proto-based nexthops

zebra,lib: command to only install proto-based nexthops

Add a command/functionality to only install proto-based nexthops.
That is nexthops owned/created by upper level protocols, not ones
implicitly created by zebra.

There are some scenarios where you would not want zebra to be
arbitrarily installing nexthop groups and but you still want
to use ones you have control over via lib/nexthop_group config
and an upper level protocol.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>