bgpd: Add support for SR-TE Policies in route-maps

bgpd: Add support for SR-TE Policies in route-maps

Example configuration:
    route-map SET_SR_POLICY permit 10
     set sr-te color 1
     !
    router bgp 1
     bgp router-id 1.1.1.1
     neighbor 2.2.2.2 remote-as 1
     neighbor 2.2.2.2 update-source lo
     address-family ipv4 unicast
      neighbor 2.2.2.2 next-hop-self
      neighbor 2.2.2.2 route-map SET_SR_POLICY in
     exit-address-family
     !
    !
Learned BGP routes from 2.2.2.2 are mapped to the SR-TE Policy
which is uniquely determined by the BGP nexthop (2.2.2.2 in this
case) and the SR-TE color in the route-map.

Co-authored-by: Renato Westphal <renato@opensourcerouting.org>
Co-authored-by: GalaxyGorilla <sascha@netdef.org>
Co-authored-by: Sebastien Merle <sebastien@netdef.org>
Signed-off-by: Sebastien Merle <sebastien@netdef.org>