bfdd: import distributed BFD implementation

bfdd: import distributed BFD implementation

Distributed BFD is a term used for BFD implementations that do not run
on the routing engine, instead it is run on a data plane (software or
hardware based).

The current code implements the basic communication between FRR BFD
daemon with an external BFD data plane and defines the protocol format
in the file `bfddp_packet.h`.

To enable/use data plane you need to start BFD daemon with the command
line `--dplaneaddr <type>:<address>`, then a socket will be opened to
listen for incoming data plane connections.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>