BigQuery: Add support for AVRO

[danoscarmike]

Refer Java issue for details - googleapis/google-cloud-java#1441

[dwsupplee]

Table::load() has AVRO support already handled, we should be good to go here.

An example:

require 'vendor/autoload.php';

use Google\Cloud\BigQuery\BigQueryClient;

$bigQuery = new BigQueryClient();
$table = $bigQuery->dataset('my_dataset')->table('my_table');

$table->load(fopen('/avro/test.avro', 'r'), [
    'jobConfig' => [
        'sourceFormat' => 'AVRO'     
    ]
]);