Implement insert & update for ActiveModel

[billy1624]

To avoid manually exec the insert ActiveModel. In below, we cannot simply save it because this is a junction table and the composite primary key should be set during insert.

sea-orm/tests/relational_tests.rs

Lines 562 to 569 in bfaa7d4

	let bobby_cakes_bakers = cakes_bakers::ActiveModel {
	cake_id: Set(chocolate_cake_res.last_insert_id as i32),
	baker_id: Set(baker_bobby_res.last_insert_id as i32),
	..Default::default()
	};
	CakesBakers::insert(bobby_cakes_bakers)
	.exec(&ctx.db)
	.await?;

Perhaps impl it here

sea-orm/sea-orm-macros/src/derives/active_model.rs

Lines 39 to 47 in 8cfa142

	impl ActiveModel {
	pub async fn save(self, db: &sea_orm::DatabaseConnection) -> Result<Self, sea_orm::DbErr> {
	sea_orm::save_active_model::<Self, Entity>(self, db).await
	}
	pub async fn delete(self, db: &sea_orm::DatabaseConnection) -> Result<sea_orm::DeleteResult, sea_orm::DbErr> {
	sea_orm::delete_active_model::<Self, Entity>(self, db).await
	}
	}

[tyt2y3]

What is the final intended API?
I kind of get the problem, but I don't get the proposed solution.