add required parameter to FromContext annotation

[groov1kk]

Added boolean required property for the @FromContext annotation.

In case of required=false and absence of a specific property in the context, the default value would be returned (null, false, 0, 0L, etc., depending on the property type) instead of ObjectNotStoredException.

For example:

// None of these values were set in context
public void myAwesomeStep(
        @FromContext(value="myStringKey", required=false) String stringValue,
        @FromContext(value="myIntKey", required=false) int intValue,
        @FromContext(value="myDoubleKey", required=false) double doubleValue,
) {
    System.out.println(stringValue); // prints null
    System.out.println(intValue); // prints 0
    System.out.println(doubleValue); // prints 0.0
}