New Dynamic Model pattern to support type-checked additional properties

Why:
The SDK code generator is being updated to improve the pattern used for
dynamic models (i.e. models that support "additionalProperties").

What:
The changes in this PR can be summarized as follows:
1) An enhanced DynamicModel class which is now parameterized with the
additionalProperties value type and includes a map for them
2) A new DynamicModelTypeAdapterFactory class which is registered with Gson
to perform serialization and deserialization of ALL generated DynamicModel
subclasses.
3) GsonSerializationHelper is now deprecated as it's no longer needed.
4) Tests related to the above functional changes.

Author: padamstx

src/main/java/com/ibm/cloud/sdk/core/service/model/DynamicModel.java

@@ -1,14 +1,103 @@
 package com.ibm.cloud.sdk.core.service.model;
 
 import java.util.HashMap;
+import java.util.Map;
+import java.util.Set;
 
+import com.google.gson.reflect.TypeToken;
 import com.ibm.cloud.sdk.core.util.GsonSingleton;
 
 /**
- * Abstract model class for objects which may have dynamic properties attached to them,
- * which is represented with an internal map.
+ * Abstract model class for a model which supports dynamic (arbitrary) properties of type T.
  */
-public abstract class DynamicModel extends HashMap<String, Object> implements ObjectModel {
+public abstract class DynamicModel<T> implements ObjectModel {
+  private TypeToken<T> additionalPropertyTypeToken;
+
+  // The set of dynamic properties associated with this object.
+  private Map<String, T> dynamicProperties = new HashMap<>();
+
+  /**
+   * Force use of 1-arg ctor.
+   */
+  @SuppressWarnings("unused")
+  private DynamicModel() {
+  }
+
+  /**
+   * This ctor accepts a TypeToken instance that represents the type of values stored in the map.
+   *
+   * @param t
+   *          the TypeToken which represents the type of map values
+   */
+  public DynamicModel(TypeToken<T> t) {
+    this.additionalPropertyTypeToken = t;
+  }
+
+  /**
+   * Returns the TypeToken which describes the type of additional properties stored in the map.
+   * @return The TypeToken which descriptes the map value type
+   */
+  public TypeToken<T> getAdditionalPropertyTypeToken() {
+    return this.additionalPropertyTypeToken;
+  }
+
+  /**
+   * Sets an arbitrary property.
+   *
+   * @param key
+   *          the name of the property to set
+   * @param value
+   *          the value of the property to be set
+   * @return the previous value of the property, or null if the property was not previously set
+   */
+  public T setProperty(String key, T value) {
+    return this.dynamicProperties.put(key, value);
+  }
+
+  /**
+   * Returns the value of the specified property.
+   *
+   * @param key
+   *          the name of the property to get
+   * @return the value of the property, or null if the property is not set
+   */
+  public T getProperty(String key) {
+    return this.dynamicProperties.get(key);
+  }
+
+  /**
+   * Returns a map containing the arbitrary properties set on this object.
+   *
+   * @return a copy of the map containing arbitrary properties set on this object
+   */
+  public Map<String, T> getProperties() {
+    return new HashMap<String, T>(this.dynamicProperties);
+  }
+
+  /**
+   * Returns the names of arbitrary properties set on this object.
+   * @return a set containing the names of arbitrary properties set on this object
+   */
+  public Set<String> getPropertyNames() {
+    return this.dynamicProperties.keySet();
+  }
+  /**
+   * Removes the specified property from this object's map of arbitrary properties.
+   *
+   * @param key
+   *          the name of the property to be removed
+   * @return the previous value of the property, or null if the property was not previously set
+   */
+  public T removeProperty(String key) {
+    return this.dynamicProperties.remove(key);
+  }
+
+  /**
+   * Removes all of the arbitrary properties set on this object.
+   */
+  public void removeProperties() {
+    this.dynamicProperties.clear();
+  }
 
   /*
    * (non-Javadoc)
@@ -23,7 +112,7 @@ public boolean equals(Object o) {
       return false;
     }
 
-    final DynamicModel other = (DynamicModel) o;
+    final DynamicModel<?> other = (DynamicModel<?>) o;
 
     return toString().equals(other.toString());
   }