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

Closes #2967. Supersedes #2972.

DeriveValueType wrappers can opt in to a generated TryGetableArray impl so
Vec<Wrapper> is readable from a SQL array column in FromQueryResult structs.
Opt-in and non-disruptive: default behaviour unchanged; the impl delegates to
the inner type's Vec support under the postgres-array feature. Includes a
Postgres round-trip test reading an int[] column into Vec<GoodId>.