Add opt-in #[sea_orm(try_getable_array)] to DeriveValueType for Vec<T> query results

[tyt2y3]

Closes #2967. Supersedes #2972.

Problem

DeriveValueType wrappers (e.g. struct GoodId(i32)) don't implement TryGetableArray, so they can't be used as Vec<GoodId> in FromQueryResult structs (Postgres arrays):

the trait bound `Vec<GoodId>: TryGetable` is not satisfied

Approach — opt-in, non-disruptive

Per the discussion on #2972, this is opt-in rather than automatic, so default behaviour is unchanged and there's no risk to existing wrappers (including nested Vec<Vec<T>> cases):

#[derive(DeriveValueType)]
#[sea_orm(try_getable_array)]
pub struct GoodId(i32);

#[derive(FromQueryResult)]
struct Row { ids: Vec<GoodId> }   // now works

• New #[sea_orm(try_getable_array)] flag generates impl TryGetableArray for GoodId, delegating to the inner type's Vec support — so it only compiles when Vec<inner>: TryGetable. Named to match the existing try_from_u64 opt-in flag (both name the trait they generate), and to avoid confusion with array_type.
• Gated on the postgres-array feature (like the existing NotU8 impl).
• Verified to compile under postgres-array + with-json (no trait-coherence conflict with the JSON TryGetableArray blanket).

Test

Postgres round-trip in value_type_tests: reads SELECT ARRAY[1,2,3]::int4[] into Vec<GoodId> via FromQueryResult.

Note: sea-orm-sync regenerates from this at release time.