Add support for built-in PostgreSQL geometric types

Closes #282

What

Adds support for PostgreSQL's built-in geometric types in entities:
point, line, lseg, box, path, polygon, circle.
These are the core Postgres geometric types (Postgres manual §8.8) — not
PostGIS, so no extension is required.

How

sqlx already provides structs for these types (PgPoint, PgPolygon, …), but
Rust's orphan rules stop us from implementing sea-orm's traits on them directly.
So this adds a thin Geo<T> newtype wrapper that implements the needed traits
(ValueType, TryGetable, Nullable, IntoActiveValue, …) and derefs to the
inner sqlx type.
Values cross the DB boundary as canonical Postgres text using the existing
save_as / select_as casts, so the core Value enum is left unchanged.
Everything is gated behind a new postgres-geometry feature.

Usage

use sea_orm::entity::prelude::*;
#[derive(Clone, Debug, PartialEq, DeriveEntityModel)]
#[sea_orm(table_name = "geo")]
pub struct Model {
    #[sea_orm(primary_key, auto_increment = false)]
    pub id: i32,
    #[sea_orm(column_type = r#"custom("point")"#, select_as = "text", save_as = "point")]
    pub location: Geo<PgPoint>,
    #[sea_orm(column_type = r#"custom("polygon")"#, select_as = "text", save_as = "polygon")]
    pub boundary: Geo<PgPolygon>,
}

Tests

Adds tests/postgres_geometry_tests.rs covering insert/select round-trips of all
seven types (including a NULL case) and native spatial queries (@>, <->).

cargo test --features sqlx-postgres,postgres-geometry --test postgres_geometry_tests