Add ImmutableConfiguration.getEnum() methods

Add ImmutableConfiguration.getEnum methods:

• org.apache.commons.configuration2.ImmutableConfiguration.getEnum(String, Class<T>)
• org.apache.commons.configuration2.ImmutableConfiguration.getEnum(String, Class<T>, T)

  /**
   * Gets an enum associated with the given configuration key.
   *
   * @param <T> The enum type whose constant is to be returned.
   * @param enumType the \{@code Class} object of the enum type from which to return a constant
   * @param key The configuration key.
   * @return The associated enum.
   *
   * @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that
   * is not a String.
   * @since 2.8
   */
   default <T extends Enum<T>> T getEnum(String key, Class<T> enumType)
  

  and

  /**
   * Gets the enum associated with the given configuration key. If the key doesn't map to an existing object, the
   * default value is returned.
   * 
   * @param <T> The enum type whose constant is to be returned.
   * @param key The configuration key.
   * @param enumType the \{@code Class} object of the enum type from which to return a constant
   * @param defaultValue The default value.
   * @return The associated enum if key is found and has valid format, default value otherwise.
   *
   * @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is
   * not a Enum.
   * @since 2.8
   */
   default <T extends Enum<T>> T getEnum(String key, Class<T> enumType, T defaultValue)