Move name constants to separate 'chains' package, to reduce dependencies

We don't want everyone that uses the `net` package to get a transitive
dependency on the Kubernetes APIs required by `npc`.

common/chains/npc.go

@@ -0,0 +1,12 @@
+package chains
+
+const (
+	MainChain    = "WEAVE-NPC"
+	DefaultChain = "WEAVE-NPC-DEFAULT"
+	IngressChain = "WEAVE-NPC-INGRESS"
+
+	EgressChain        = "WEAVE-NPC-EGRESS"
+	EgressDefaultChain = "WEAVE-NPC-EGRESS-DEFAULT"
+	EgressCustomChain  = "WEAVE-NPC-EGRESS-CUSTOM"
+	EgressMarkChain    = "WEAVE-NPC-EGRESS-ACCEPT"
+)