Add API and beans for User and UserManager.

plugin/src/main/java/com/alttd/essentia/user/EssentiaUserManager.java

@@ -0,0 +1,53 @@
+package com.alttd.essentia.user;
+
+import org.bukkit.entity.Player;
+
+import java.util.Map;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+
+public class EssentiaUserManager implements UserManager {
+
+    private final Map<UUID, User> essentiaPlayers = new ConcurrentHashMap<>();
+
+    @Override
+    public User getUser(Player player) {
+        return getUser(player.getUniqueId());
+    }
+
+    @Override
+    public User getUser(UUID uuid) {
+        User user = essentiaPlayers.get(uuid);
+        if (user != null) {
+            return user;
+        } else {
+            return createNewUser(uuid);
+        }
+    }
+
+    @Override
+    public void addUser(User user) {
+        essentiaPlayers.put(user.getUUID(), user);
+    }
+
+    @Override
+    public void removeUser(UUID uuid) {
+        essentiaPlayers.remove(uuid);
+    }
+
+    @Override
+    public boolean hasUser(UUID uuid) {
+        return essentiaPlayers.containsKey(uuid);
+    }
+
+    @Override
+    public Map<UUID, User> getUsers() {
+        return essentiaPlayers;
+    }
+
+    @Override
+    public User createNewUser(UUID uuid) {
+        return null;
+    }
+
+}