feat: PlantDB types and interfaces

Created the types and interfaces relating to the PlantDatabase

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

@@ -0,0 +1,12 @@
+package ch.zhaw.gartenverwaltung.io;
+
+import ch.zhaw.gartenverwaltung.types.Plant;
+import ch.zhaw.gartenverwaltung.types.HardinessZone;
+
+import java.util.List;
+import java.util.Optional;
+
+public interface PlantDatabase {
+    List<Plant> getPlantList(HardinessZone zone);
+    Optional<Plant> getPlantById(long id);
+}