fix: set primary key as string

[zhanang19]

When eager loading relationship using PostgreSQL, like this Outlet::with('district')->first(), the following error occured:

Illuminate\Database\QueryException  SQLSTATE[42883]: Undefined function: 7 ERROR:  operator does not exist: character = integer
LINE 1: ...sia_districts" where "indonesia_districts"."code" in (352314...
                                                             ^
HINT:  No operator matches the given name and argument types. You might need to add explicit type casts. (Connection: pgsql, SQL: select * from "indonesia_districts" where "indonesia_districts"."code" in (352314)).

After configuring the model primary key as string resolve the issue.

Reference: https://laravel.com/docs/11.x/eloquent#primary-keys