sql/dialect/schema: support setting PK to serial types in postgres

[a8m]

Fixed #2744.

Users that want to use a serial type for their id field, can create one mixed-in schema (with the definition below) and embed it in all schemas.

func (MixinID) Fields() []ent.Field {
	return []ent.Field{
		field.Int("id").
			SchemaType(map[string]string{
				dialect.Postgres: postgres.TypeBigSerial,
			}),
	}
}