bgpd: Add RFC9234 implementation

bgpd: Add RFC9234 implementation

RFC9234 is a way to establish correct connection roles (Customer/
Provider, Peer or with RS) between bgp speakers. This patch:
- Add a new configuration/terminal option to set the appropriate local
role;
- Add a mechanism for checking used roles, implemented by exchanging
the corresponding capabilities in OPEN messages;
- Add strict mode to force other party to use this feature;
- Add basic support for a new transitive optional bgp attribute - OTC
(Only to Customer);
- Add logic for default setting OTC attribute and filtering routes with
this attribute by the edge speakers, if the appropriate conditions are
met;
- Add two test stands to check role negotiation and route filtering
during role usage.

Signed-off-by: Eugene Bogomazov <eb@qrator.net>