Execution context property declarations and property metadata attributes

Make it possible to associate metatada with each property in the execution context.

Add two methods to ExecutionContext:

/**
 * Declare a property in the execution context.
 *
 * @param property The property to declare.
 */
public void declareProperty(ExecutionContextProperty property)

/**
 * @param key the key under which is stored the property to retrieve
 * @return {@code true} if there is a property declared for the given key.
 */
public boolean hasProperty(String key)

An execution context property contains the following metadata, that controls how the property value should be treated when managing execution contexts:

• Read-only - The value may not be updated within the the execution context.
• Inherited - The property will be inherited from the current context when replacing the execution context within the scope of a request.
• Clone - Also clone the value when the execution context is cloned.
• Type - The class of the value, for typechecking when replacing the value.
• Non-null - The value may not be null