Support internally tagged enums

[jfizz]

As described in the serde docs: https://serde.rs/enum-representations.html#internally-tagged.

The current behavior

#[derive(Deserialize, Serialize, Debug, Clone)]
pub struct ProductItem {
    pub details: ProductItemDetails,
}

#[derive(Deserialize, Serialize, Debug, Clone)]
#[serde(tag = "type")]
#[serde(rename_all = "snake_case")]
pub enum ProductItemDetails {
    Card(CardDetails),
    Gift(GiftDetails),
}

#[derive(Deserialize, Serialize, Debug, Clone)]
pub struct GiftDetails {
    pub for_item: usize,
}

...

which results in

unknown variant `type`, expected one of `card`, `gift`

For reference, the DynamoDB item would look like (in JSON format):

{
  "details": {
    "type": "gift",
    "for_item": 0
  }
}