implement SBFD

[forrestchu]

implementing the SBFD feature (RFC7880, RFC7881) in FRR.

What is the motivation for this PR?
The PhoenixWing project aims to implement SRv6 features into the SONiC community. In PhoenixWing traffic engineering case, we use SBFD to protect SRv6 TE paths.

How did you do it?
SBFD HLD in SoNiC community: sonic-net/SONiC#1766

use SBFD to protect TE path, two types of configs are supported:

1. SBFD echo mode, which mainly used in Our SRv6 TE case. Only need to config at local side:
   configure terminal->
   bfd ->
   peer X::X bfd-mode sbfd-echo bfd-name name local-address X::X encap-type SRv6 encap-data X::X source-ipv6 X::X
2. SBFD initiator and reflector mode, Need to config at local side and remote side:
   2.1) local config:
   configure terminal->
   bfd ->
   peer X::X bfd-mode sbfd-init bfd-name name local-address X::X encap-type SRv6 encap-data X::X source-ipv6 X::X remote-discr 12345
   2.2) remote config:
   configure terminal ->
   bfd ->
   sbfd reflector source-address X::X discriminator 12345

[riw777]

looks good ... I think we need a topo test for this, though

[riw777]

Needs to be broken into multiple commits, and this also needs a topo test.

[donaldsharp]

a) This needs to be broken up into multiple commits. 4k lines to review is impossible. Break it down into small logical bits of work, this will never be reviewed otherwise
b) Actually take the time and write a topotest to show that this works.
c) The commit message is utterly useless and will help no-one in the future understand what is going on. This needs to be addressed
d) There is no documentation. This must be added as well.

Without some major changes this is dead in the water.

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.