Added MeetingState to symbolize limited options

oparl-server/src/main/java/de/twomartens/oparlservice/entity/MeetingState.java

@@ -0,0 +1,17 @@
+package de.twomartens.oparlservice.entity;
+
+import com.fasterxml.jackson.annotation.JsonValue;
+import lombok.Getter;
+import lombok.RequiredArgsConstructor;
+import lombok.ToString;
+
+@RequiredArgsConstructor
+@Getter(onMethod_={@JsonValue})
+@ToString
+public enum MeetingState {
+    SCHEDULED("terminiert"),
+    INVITED("eingeladen"),
+    FINISHED("durchgeführt");
+
+    private final String value;
+}