pkg/mesh,pkg/wireguard: allow DNS name endpoints

This commit allows DNS names to be used when specifying the endpoint
for a node in the WireGuard mesh. This is useful in many scenarios, in
particular when operating an IoT device whose public IP is dynamic. This
change allows the administrator to use a dynamic DNS name in the node's
endpoint.

One of the side-effects of this change is that the WireGuard port can
now be specified individually for each node in the mesh, if the
administrator wishes to do so.

*Note*: this commit introduces a breaking change; the
`force-external-ip` node annotation has been removed; its functionality
has been ported over to the `force-endpoint` annotation. This annotation
is documented in the annotations.md file. The expected content of this
annotation is no longer a CIDR but rather a host:port. The host can be
either a DNS name or an IP.

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

pkg/wireguard/conf_test.go

@@ -95,6 +95,28 @@ func TestCompareConf(t *testing.T) {
 		`),
 			out: false,
 		},
+		{
+			name: "different value",
+			a: []byte(`[Interface]
+		PrivateKey = private
+		ListenPort = 51820
+
+		[Peer]
+		Endpoint = 10.1.0.2:51820
+		PublicKey = key
+		AllowedIPs = 10.2.2.0/24, 192.168.0.1/32, 10.2.3.0/24, 192.168.0.2/32, 10.4.0.2/32
+		`),
+			b: []byte(`[Interface]
+		PrivateKey = private
+		ListenPort = 51820
+
+		[Peer]
+		Endpoint = 10.1.0.2:51820
+		PublicKey = key2
+		AllowedIPs = 10.2.2.0/24, 192.168.0.1/32, 10.2.3.0/24, 192.168.0.2/32, 10.4.0.2/32
+		`),
+			out: false,
+		},
 		{
 			name: "section order",
 			a: []byte(`[Interface]