refactor: change private properties to protected for extensibility

[aniruddha-adhikary]

Thanks for building AdminJS!

I ran into a little snag when trying to extend some of the Prisma resource functionalities, specifically with the buildSortBy, model... properties being private:

class PermissionCheckedResource extends Prisma.Resource {
  findOne(...) {
    this.buildSortBy
     ^ this method is private thus cannot update behavior easily
  }
}

Having this method as private means a lot of repeating code if we try to customize behavior. It would be awesome if it could be switched to protected to make subclassing a breeze and keep our codebase clean.

Thought this might make AdminJS more flexible for others too who might be looking to extend functionality in similar ways.

Thanks for considering this tweak!