Add --service-environment to allow testing against sandbox environment

src/main/java/org/asamk/signal/ServiceEnvironmentCli.java

@@ -0,0 +1,16 @@
+package org.asamk.signal;
+
+public enum ServiceEnvironmentCli {
+    LIVE {
+        @Override
+        public String toString() {
+            return "live";
+        }
+    },
+    SANDBOX {
+        @Override
+        public String toString() {
+            return "sandbox";
+        }
+    },
+}