[4.x] Improve support for custom laravel-excel export class using collection.

[yajra]

This PR will improve support for the laravel-excel package export class.

1. All rows will now be included instead of the filtered values displayed in the table. (Potential breaking change)
2. Add DataTablesCollectionExport abstract class.

Example Usage

1. Create an export class php artisan make:export UsersExport
2. Update the generated export class and extend DataTablesCollectionExport

namespace App\Exports;

use Yajra\DataTables\Exports\DataTablesCollectionExport;

class UsersExport extends DataTablesCollectionExport
{
    
}

3. Update your UsersDataTable class and set protected $exportClass = UsersExport::class

class UsersDataTable extends DataTable
{
    protected $exportClass = UsersExport::class;

4. Update your export class as needed. See official package docs: https://docs.laravel-excel.com/3.1/exports/collection.html

Example Export Class

<?php

namespace App\Exports;

use Maatwebsite\Excel\Concerns\WithMapping;
use Yajra\DataTables\Exports\DataTablesCollectionExport;

class UsersExport extends DataTablesCollectionExport implements WithMapping
{
    public function headings(): array
    {
        return [
            'Name',
            'Email',
        ];
    }

    public function map($row): array
    {
        return [
            $row['name'],
            $row['email'],
        ];
    }
}