bgp: enhance defaulting of bgp routes command

[rastislavs]

After simplification of cilium-dbg bgp routes advertised command in cilium/cilium#30033 is merged, we can rely on it to make the same simplifications in cilium CLI:

• allow retrieving advertised routes for each peer (without the need to specify a peer)
• introduce defaulting for AFI & SAFI

Examples:

$ cilium bgp routes                                                                                                                                                                      ✔ 
(Defaulting to `available ipv4 unicast` routes, please see help for more options)

Node                                  VRouter   Prefix          NextHop   Age   Attrs
clab-bgp-cplane-devel-control-plane   65001     10.244.0.0/24   0.0.0.0   3s    [{Origin: i} {Nexthop: 0.0.0.0}]   

$ cilium bgp routes advertised                                                                                                                                                           ✔ 
(Defaulting to `ipv4 unicast` AFI & SAFI, please see help for more options)

Node                                  VRouter   Peer        Prefix          NextHop     Age   Attrs
clab-bgp-cplane-devel-control-plane   65001     172.0.0.1   10.244.0.0/24   172.0.0.2   8s    [{Origin: i} {AsPath: 65001} {Nexthop: 172.0.0.2}]   

$ cilium bgp routes advertised ipv4                                                                                                                                                      ✔ 
(Defaulting to `unicast` SAFI, please see help for more options)

Node                                  VRouter   Peer        Prefix          NextHop     Age   Attrs
clab-bgp-cplane-devel-control-plane   65001     172.0.0.1   10.244.0.0/24   172.0.0.2   18s   [{Origin: i} {AsPath: 65001} {Nexthop: 172.0.0.2}]   

[harsimran-pabla]

lgtm, thank you 🙏