Add "antctl get bgppeers" agent command

[Atish-iaf]

Add antctl get bgppeers agent command to print current status of all BGP peers
of effective BGP policy applied on the local Node.

For #6209

[antoninbas]

Can we merge the address and the port here? E.g., 192.168.77.201:179

[antoninbas]

This is a brittle way to do this and not idiomatic. You need to define a custom error and check it by name here. There are a few examples of this in our code base, such as ErrExternalIPPoolNotFound in the ExternalIPPool controller.

[antoninbas]

The GetBGPPeerStatus function should take a context as parameter, which you can can forward here. There is no reason to use context.TODO() here.

[antoninbas]

holding a read lock while calling this function may be bit overkill. In the case of gobgp at least, all these operations are asynchronous (so there can be some delay, hence the context parameter) and all the server functions can be called concurrently as far as I know.

so the following would probably be correct:

getBgpServer := func() bgp.Interface {
        c.bgpPolicyStateMutex.RLock()
	defer c.bgpPolicyStateMutex.RUnlock()
        if c.bgpPolicyState == nil {
		return nil
	}
	return c.bgpPolicyState.bgpServer
}

bgpServer := getBgpServer()
if bgpServer == nil {
	// return error
}
peers, err := bgpServer.GetPeers(ctx)

This way we don't hold the read lock for a potentially expensive async operation.
That means that we have to guarantee that all bgp.Interface implementations (at the moment we only have gobgp) support concurrent function calls. Probably worth double checking that this assumption holds for gobgp, but it seems to me that all API calls get serialized to a channel.