new feature - update_item

[mateuszboryn]

Please add support for update_item:

@dynamoclass(...)
class Item:
    key: str
    field1: str
    field2: str
    field3: str

item.update_item(fields=['field1', 'field2'])

code above would update only field1 and field2 and not field3 nor any other field that is not defined in class definition.