Output json when receiving messages from dbus and --json parameter is given

Fixes #138

src/main/java/org/asamk/signal/manager/JsonStickerPack.java

@@ -0,0 +1,29 @@
+package org.asamk.signal.manager;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+import java.util.List;
+
+class JsonStickerPack {
+
+    @JsonProperty
+    public String title;
+
+    @JsonProperty
+    public String author;
+
+    @JsonProperty
+    public JsonSticker cover;
+
+    @JsonProperty
+    public List<JsonSticker> stickers;
+
+    public static class JsonSticker {
+
+        @JsonProperty
+        public String emoji;
+
+        @JsonProperty
+        public String file;
+    }
+}