feat: GardenPlan types and interfaces

Created the type and interface relating to the GardenPlan

src/main/java/ch/zhaw/gartenverwaltung/io/GardenPlan.java

@@ -0,0 +1,12 @@
+package ch.zhaw.gartenverwaltung.io;
+
+import ch.zhaw.gartenverwaltung.types.UserPlanting;
+
+import java.io.IOException;
+import java.util.List;
+
+public interface GardenPlan {
+    List<UserPlanting> getPlantings();
+    void savePlanting(UserPlanting planting) throws IOException;
+    void removePlanting(UserPlanting planting) throws IOException;
+}

src/main/java/ch/zhaw/gartenverwaltung/types/UserPlanting.java

@@ -0,0 +1,10 @@
+package ch.zhaw.gartenverwaltung.types;
+
+import java.util.Date;
+
+public record UserPlanting(
+        long plantId,
+        Date startDate,
+        int area
+) {
+}