Allow fallback to IPv6 if private network is not available

Related: #77
Signed-off-by: Richard Kosegi <richard.kosegi@gmail.com>

src/main/java/cloud/dnation/jenkins/plugins/hetzner/launcher/DefaultV6ConnectionMethod.java

@@ -0,0 +1,47 @@
+/*
+ *     Copyright 2024 https://dnation.cloud
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *          http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package cloud.dnation.jenkins.plugins.hetzner.launcher;
+
+import cloud.dnation.hetznerclient.ServerDetail;
+import cloud.dnation.jenkins.plugins.hetzner.Messages;
+import edu.umd.cs.findbugs.annotations.NonNull;
+import hudson.Extension;
+import hudson.model.Descriptor;
+import lombok.NoArgsConstructor;
+import org.jenkinsci.Symbol;
+import org.kohsuke.stapler.DataBoundConstructor;
+
+@NoArgsConstructor(onConstructor = @__({@DataBoundConstructor}))
+public class DefaultV6ConnectionMethod extends AbstractConnectionMethod {
+    @Override
+    public String getAddress(ServerDetail server) {
+        if (server.getPrivateNet() != null && !server.getPrivateNet().isEmpty()) {
+            return server.getPrivateNet().get(0).getIp();
+        } else {
+            return server.getPublicNet().getIpv6().getIp();
+        }
+    }
+
+    @Extension
+    @Symbol("defaultV6")
+    public static final class DescriptorImpl extends Descriptor<AbstractConnectionMethod> {
+        @NonNull
+        @Override
+        public String getDisplayName() {
+            return Messages.connection_method_defaultV6();
+        }
+    }
+}

src/main/resources/cloud/dnation/jenkins/plugins/hetzner/Messages.properties

@@ -25,6 +25,7 @@ primaryip.default=Use default behavior
 primaryip.bylabelselector.failing=Allocate primary IPv4 using label selector, fail if none is available
 primaryip.bylabelselector.ignoring=Allocate primary IPv4 using label selector, ignore any error
 connection.method.default=Connect using private IPv4 address if available, otherwise using public IPv4 address
+connection.method.defaultV6=Connect using private IPv4 address if available, otherwise using public IPv6 address
 connection.method.public=Connect using public IPv4 address only
 connection.method.publicV6=Connect using public IPv6 address only
 connectivity.private-only=Only private networking will be used. Network ID or label expression must be provided as well.