graphclient is missing a BatchRequestRequestBuilder

[andrueastman]

Taking a look at the example at

https://github.com/microsoftgraph/msgraph-sdk-php/blob/7fa86c054e315c5d6e081f3f1f3b963562c923f3/docs/Examples.md?plain=1#L658

The graph client is missing the setup of the BatchRequestRequestBuilder.

The code of the graphclient should have an batch() function returning the BatchRequestRequestBuilder so that the example/sample is simplified to

$batchResponse = $graphServiceClient->batch()->postAsync($batchRequestContent)->wait();