Provide converters for AttributeValueUpdate

[behos]

This will allow us to have stronly-typed updates

Not sure how we'd prefer to create it. We could offer an impl for each DynamoDBItem argument

#[derive(DynamoDBItem)]
struct MyItem {
    a: i32,
    b: String
}

impl MyItem {
    fn update_a(&self, operation: Option<AttributeAction>, value: i32) -> AttributeValueUpdate {
        ...
    }

    fn update_b(&self, operation: Option<AttributeAction>, value: String) -> AttributeValueUpdate {
        ...
    }
}

Alternatively we can return pairs of (String, AttributeValueUpdate) where String is the field name. We can then collect all of those updates into a HashMap to use for updates