#[derive(FromRow)] should support columns which may be omitted from the result set

[abonander]

We should provide the ability to mark fields with #[sqlx(defaultl)] which allows them to be skipped if they're not in the result set:

https://discordapp.com/channels/665528275556106240/665528275556106243/708410674748981309

#[derive(sqlx::FromRow, Serialize)]
struct MyEntity {
    ID: String,
    #[serde(skip_serializing_if = "Option::is_none")]
    #[sqlx(default)] // if column is not set, uses `Default::default()` instead
    age: Option<i64>, // If this column doesn't appear in the result set, I want it to be None
}

Corresponds to #[serde(default)]: https://serde.rs/field-attrs.html#default