Add a way to filter dataloader query

[ccatterina]

Hi,
I've added a new decorator calledFilteredTypeormLoader which works exactly as TypeormLoader but it applies a custom function to the query builder.

I've implemented this functionality in order to apply some authorization filters on the query created by the TypeormLoader.

Example

@Entity()
@ObjectType()
export class User extends BaseEntity {
  @PrimaryGeneratedColumn()
  @Field()
  id: number;

  @ManyToMany(() => Customer, (customer) => customer.users)
  @JoinTable()
  @Field((_type) => [Customer])
  @FilteredTypeormLoader((qb, context) => {
    if (!context.currentUser.isAdmin) {
      qb.andWhere(...)
    }
  })
  customers: Promise<Customer[]>;

  ...
}

I can add some tests and an usage example if you want to include this functionality to the package, otherwise feel free to close the PR.