Allow setting defaults in dynamodb items

[behos]

Setting default values will allow backwards incompatible migrations for existing items without the need to define optional new fields.

Item stored in ddb:

{
   "id": "1"
   "name": "hello"
}

Current ddb item:

#[derive(DynamoDBItem)]
struct Reviewer {
    #[hash] id: u16,
    name: String
}

You should be able to update the struct with a new field providing a default:

#[derive(DynamoDBItem)]
struct Reviewer {
    #[hash] id: u16,
    name: String,
    #[default=0] reviews: u16
}

When converting from the dynamodb item, if the value is missing, the default should be used.