lib: add a new northbound plugin for Sysrepo

lib: add a new northbound plugin for Sysrepo

This plugin leverages the northbound API to integrate FRR with Sysrepo,
a YANG-based configuration and operational state data store.

The plugin is linked to the libsysrepo library and communicates with
the sysrepod daemon using GPB (Google Protocol Buffers) over AF_UNIX
sockets. The integration consists mostly of glue code that calls the
appropriate FRR northbound callbacks in response to events triggered
by the sysrepod daemon (e.g. request to change the configuration or to
fetch operational data).

To build the sysrepo plugin, provide the --enable-sysrepo option to the
configure script while building FRR (the libsysrepo library needs to be
installed in the system).

When installed, the sysrepo plugin will be available for all FRR daemons
and can be loaded using the -M (or --module) command line option.

Example: bgpd -M sysrepo.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>