Feature Request: Add support for `sqlx(flatten)`:

[poliorcetics]

Right now atmosphere doesn't support sqlx(flatten) which prevents lots of code reuse.

I would like the following test to work transparently (or maybe needing an sql(flattened) attribute of its own, that's fine):

CREATE TABLE scientists (
    id     INT PRIMARY KEY,
    name   TEXT NOT NULL,
    domain TEXT NOT NULL
);

use atmosphere::prelude::*;
use atmosphere_core::Table;
use sqlx::prelude::{FromRow, Type};

#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone)]
#[table(name = "scientists", schema = "public")]
struct ScientistEntry {
    #[sql(pk)]
    id: i32,
    #[sqlx(flatten)]
    scientist: Scientist,
}

#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone, FromRow, Type)]
struct Scientist {
    name: String,
    domain: String,
}

impl Scientist {
    pub fn new(name: &str, domain: &str) -> Self {
        Self {
            name: name.into(),
            domain: domain.into(),
        }
    }
}

#[sqlx::test(migrations = "tests/flatten/migrations")]
async fn create(pool: sqlx::PgPool) {
    atmosphere::testing::create(
        &pool,
        ScientistEntry {
            id: 0,
            scientist: Scientist::new("Rosalyn Yalow", "physiology and medicine"),
        },
    )
    .await;

    ScientistEntry {
        id: 1,
        scientist: Scientist::new("Chien-Shiung Wu", "physics"),
    }
    .upsert(&pool)
    .await
    .unwrap();
}

#[sqlx::test(migrations = "tests/flatten/migrations")]
async fn read(pool: sqlx::PgPool) {
    atmosphere::testing::read(
        &pool,
        ScientistEntry {
            id: 0,
            scientist: Scientist::new("Fanny Bullock Workman", "geography and exploration"),
        },
    )
    .await;
}

#[sqlx::test(migrations = "tests/flatten/migrations")]
async fn update(pool: sqlx::PgPool) {
    atmosphere::testing::update(
        &pool,
        ScientistEntry {
            id: 0,
            scientist: Scientist::new("Elisabeth S. Vrba", "paleontology"),
        },
        vec![
            ScientistEntry {
                id: 0,
                scientist: Scientist::new("Lydia Villa-Komaroff", "molecular biology"),
            },
            ScientistEntry {
                id: 0,
                scientist: Scientist::new("Trota of Salerno", "medicine"),
            },
        ],
    )
    .await;
}

#[sqlx::test(migrations = "tests/flatten/migrations")]
async fn delete(pool: sqlx::PgPool) {
    atmosphere::testing::delete(
        &pool,
        ScientistEntry {
            id: 0,
            scientist: Scientist::new("Sheila Tobias", "mathematics and science education"),
        },
    )
    .await;
}