extend pg lower function to support Multirange

[guissalustiano]

Extending the work of #4159, this PR updates one Range function which also support multi-range.
This is a small PR to check the format, once approved I'm going to implement for the other methods.
I also add more example testing nullability and simplify the example removing the table and only use the select.

[guissalustiano]

The autotype test is broken

error[E0107]: type alias takes 2 generic arguments but 1 generic argument was supplied
   --> diesel_derives/tests/auto_type.rs:395:9
    |
395 |         lower(pg_extras::range),
    |         ^^^^^ ---------------- supplied 1 generic argument
    |         |
    |         expected 2 generic arguments
    |
note: type alias defined here, with 2 generic parameters: `R`, `T`
   --> /home/guiss/oss/diesel/diesel/src/pg/expression/helper_types.rs:314:10
    |
314 | pub type lower<R, T> = super::functions::lower<SqlTypeOf<R>, SqlTypeOf<T>, R>;
    |          ^^^^^ -  -
help: add missing generic argument
    |
395 |         lower(pg_extras::range, T),

I tried to follow the array_append example but it didn't work. Could someone help-me with that?

[weiznich]

Suggested change

	fn lower<Rang: MultirangeOrRangeMaybeNullable<Inner=T> + SingleValue, T: SingleValue>(range: Rang) -> Nullable<T>;
	fn lower<Rang: MultirangeOrRangeMaybeNullable + SingleValue>(range: Rang) -> Nullable<Rang::Inner>;

We should be able to remove the T type here in favor of using the associated type as return type directly. That should hopefully also allow the type lower to be unchanged and remain compatible with #[auto_type]

[guissalustiano]

Thanks 🙏🏽 !