bgpv1: Implement AdvertisedPathAttributes for CiliumBGPNeighbor using BGP routing policies

[rastislavs]

This PR adds a new field AdvertisedPathAttributes into the CiliumBGPNeighbor part of the CiliumBGPPeeringPolicy CRD, which can be used to apply additional NLRI attributes (communities / local preference) to matched routes when advertising them to a BGP peer.

The implementation relies on the GoBGP routing policies, which are internally abstracted to a new RoutePolicy type and new Router interface methods AddRoutePolicy + RemoveRoutePolicy.

For now, we support this for routes advertised for k8s LB services (matching by CiliumLoadBalancerIPPools) and Pod CIDRs (matching by CiliumNodes).

For more details, please see the individual commits.

Example:

CiliumBGPPeeringPolicy configuration:

apiVersion: cilium.io/v2alpha1
kind: CiliumBGPPeeringPolicy
spec:
  virtualRouters: 
  - localASN: 65000
    exportPodCIDR: true
    serviceSelector:
      matchLabels:
        advertise: bgp
    neighbors:
    - peerASN: 65000
      peerAddress: 172.0.0.1/32
      advertisedPathAttributes:
      # add community attribute to all routes advertised for the matched CiliumLoadBalancerIPPool
      - selectorType: CiliumLoadBalancerIPPool
        selector:
          matchLabels:
            environment: production
        communities:
          standard:
          - 65001:100
      # add local preference & community attributes to Pod CIDR routes of all nodes
      - selectorType: PodCIDR
        localPreference: 150
        communities:
          standard:
          - 65001:150

Matching a LB pool:

apiVersion: "cilium.io/v2alpha1"
kind: CiliumLoadBalancerIPPool
metadata:
  name: "cilium-pool"
  labels:
    environment: production
spec:
  cidrs:
  - cidr: "192.168.100.0/24"

Causes adding a Community and setting appropriate Local Preference on advertised routes (10.244.0.0/24 is a pod CIDR and 192.168.100.190/32 is a k8s service VIP):

$ cilium bgp routes advertised ipv4 unicast peer 172.0.0.1

VRouter   Prefix               NextHop     Age     Attrs
65000     10.244.0.0/24        172.0.0.2   3m31s   [{Origin: i} {LocalPref: 150} {Communities: 65001:150}] {Nexthop: 172.0.0.2}
65000     192.168.100.190/32   172.0.0.2   3m32s   [{Origin: i} {LocalPref: 100} {Communities: 65001:100}] {Nexthop: 172.0.0.2} 

(Note that Local Preference attribute is sent only to iBGP peers and defaults to 100)

On the peered router:

tor# sh bgp ipv4 10.244.0.0/24
BGP routing table entry for 10.244.0.0/24, version 14
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Local
    172.0.0.2 from 172.0.0.2 (172.0.0.2)
      Origin IGP, localpref 150, valid, internal, best (First path received)
      Community: 65001:150
      Last update: Mon Aug 28 09:13:16 2023

tor# sh bgp ipv4 192.168.100.190/32
BGP routing table entry for 192.168.100.190/32, version 13
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Local
    172.0.0.2 from 172.0.0.2 (172.0.0.2)
      Origin IGP, localpref 100, valid, internal, best (First path received)
      Community: 65001:100
      Last update: Mon Aug 28 09:13:16 2023

Release Note:

Implement `AdvertisedPathAttributes` for `CiliumBGPNeighbor` in the `CiliumBGPPeeringPolicy` CRD to allow setting BGP Community and Local Preference path attributes for advertised BGP routes.

[ldelossa]

Some questions to clarify, but overall code and structure looks good.

[YutaroHayakawa]

I did an initial look. Thanks! This is cool! I have several questions.

[rastislavs]

/test

[YutaroHayakawa]

My concerns are addressed. LGTM 👍

[harsimran-pabla]

Great work!!

[rastislavs]

/test

[danehans]

This is a really nice feature. I have a few nits that can be resolved in a follow-up PR.