feat: add metadataCallback to CallOptions

[Hectorhammett]

Currently there is no way to get the response headers in an simple way from the user's perspective:

$response = $someClient->someMethod($request);
var_dump($response)// This is just a Proto object, we do not have access to the headers

The gRPC transport supports the metadataCallback callback. This PR allows the user to pass this callback to the CallOptions so the headers are easily accessible:

$responseHeaders = null;
$response = $someClient->someMethod($request, [
    'metadataCallback' => function (array $headers) use (&$responseHeader){
        $responseHeaders = $headers;
    }
])

var_dump($response) // The Proto object
var_dump($responseHeaders) // The response headers in an array