Move toNode, add type helpers, split peers and shared

This commit moves toNode to the bottom of the file, and adds a helper
function for lists of Machines to be converted.

It also adds string helpers for Machines and lists of machines.

Lastly it splits getPeers into getDirectPeers, which exist in the same
namespace, and getShared, which is nodes shared with the namespace.

getPeers is kept as a function putting together the two lists for
convenience.

api.go

@@ -242,11 +242,20 @@ func (h *Headscale) getMapResponse(mKey wgkey.Key, req tailcfg.MapRequest, m *Ma
 		DisplayName: m.Namespace.Name,
 	}
 
+	nodePeers, err := peers.toNodes(true)
+	if err != nil {
+		log.Error().
+			Str("func", "getMapResponse").
+			Err(err).
+			Msg("Failed to convert peers to Tailscale nodes")
+		return nil, err
+	}
+
 	resp := tailcfg.MapResponse{
 		KeepAlive: false,
 		Node:      node,
-		Peers:     *peers,
-		//TODO(kradalby): As per tailscale docs, if DNSConfig is nil,
+		Peers:     nodePeers,
+		// TODO(kradalby): As per tailscale docs, if DNSConfig is nil,
 		// it means its not updated, maybe we can have some logic
 		// to check and only pass updates when its updates.
 		// This is probably more relevant if we try to implement