Add financial transaction classes

Patchwork/src/main/java/me/totalfreedom/economy/CompletedTransaction.java

@@ -0,0 +1,8 @@
+package me.totalfreedom.economy;
+
+public interface CompletedTransaction
+{
+    Transaction getTransaction();
+
+    TransactionResult getResult();
+}

Patchwork/src/main/java/me/totalfreedom/economy/Transaction.java

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+package me.totalfreedom.economy;
+
+public interface Transaction
+{
+    Transaction copy();
+
+    EconomicEntity getSource();
+
+    EconomicEntity getDestination();
+
+    long getTransferAmount();
+
+    long addBalance(final long amount);
+
+    long removeBalance(final long amount);
+}

Patchwork/src/main/java/me/totalfreedom/economy/TransactionResult.java

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+package me.totalfreedom.economy;
+
+import net.kyori.adventure.text.Component;
+
+public interface TransactionResult
+{
+    String getMessage();
+
+    boolean isSuccessful();
+
+    Component getComponent();
+}