This adds first-party row-level mapping support to tokio-postgres.

postgres-types::FromSql already handles decoding one Postgres cell into one Rust value. This change set adds the missing row-level layer: mapping a Row onto a user struct by generating the same row.try_get("field")? boilerplate users commonly write by hand.

This is intentionally not an ORM and does not use serde as the decoding contract. It preserves the existing FromSql behavior for UUID, chrono, jiff, JSON, arrays, custom types, and other feature-gated decoders.

This is how it can be useful:

use tokio_postgres::FromRow;

#[derive(FromRow)]
struct User {
    id: i64,
    name: String,
}

let users = rows
    .iter()
    .map(User::from_row)
    .collect::<Result<Vec<_>, _>>()?;

And, for owned mappings, this also adds convenience query methods:

let user = client
    .query_one_as::<User>("SELECT id, name FROM users WHERE id = $1", &[&id])
    .await?;

let users = client
    .query_as::<User>("SELECT id, name FROM users ORDER BY id", &[])
    .await?;

Borrowed mapping remain supported through direct FromRow::from_row(&row). The query_*_as convenience methods return owned values and intentionally cannot be used with row-borrowing mappings.

All in all, this covers a common slice of ORM ergonomics while staying inside the existing tokio-postgres decoding model.