yang, ripd: add 'frr-ripd.yang' and associated stub callbacks

yang, ripd: add 'frr-ripd.yang' and associated stub callbacks

Introduce frr-ripd.yang, which defines a model for managing the FRR
ripd daemon. Also add frr-route-types.yang which defines typedefs for
FRR route types.

Update the 'frr_yang_module_info' array of ripd with the new 'frr-ripd'
module.

Add two new files (rip_cli.[ch]) which should contain all ripd commands
converted to the new northbound model. Centralizing all commands in a
single place will facilitate the process of moving the CLI to a separate
program in the future.

Add automatically generated stub callbacks in rip_northbound.c. These
callbacks will be implemented gradually in the following commits.

Add example JSON/XML ripd configurations in yang/examples/.

Add the confd.frr-ripd.yang YANG module with annotations specific to
the ConfD daemon.

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