bgpd: Pass non-transitive ext-communities between Route Server and RS clients

bgpd: Pass non-transitive ext-communities between Route Server and RS clients

https://datatracker.ietf.org/doc/html/rfc7947#section-2.2

Optional recognized and unrecognized BGP attributes,
   whether transitive or non-transitive, SHOULD NOT be updated by the
   route server (unless enforced by local IXP operator configuration)
   and SHOULD be passed on to other route server clients.

By default LB ext-community works with iBGP peers. When we receive a route
from eBGP peer, we can send LB ext-community to iBGP peers.

With this patch, allow sending LB ext-community to iBGP/eBGP peers if they
are set as RS clients.

FRR does not send non-transitive ext-communities to eBGP peers, but for
example GoBGP sends and if it's set as RS client, we should pass those attributes
towards another RS client.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>