Refctor Solver.java to extend AbstractPointer (#10064)

This refactors the Solver class in the java API to extend AbstractPointer similar to other cvc5 classes.
It also cleans up redundant code for Abstract pointers. and adds Context.deletePointers to java examples as mentioned in issue #10052.

examples/api/java/QuickStart.java

@@ -195,5 +195,6 @@ public class QuickStart
       }
       //! [docs-java-quickstart-15 end]
     }
+    Context.deletePointers();
   }
 }