Support for Compute V2 Operations Client in OperationsResponse

[bshaffer]

We need to modify the way the additional args are passed in from GAX. For example, in Compute's descriptor_config.php, we have the following:

'SomeRPCMethod' => [
    'longRunning' => [
        'additionalArgumentMethods' => [
            'getProject',
            'getZone',
        ],
        // ...
    ]
]

This is then passed into the OperationResponse class as $additionalArgs by GAX:

gax-php/src/GapicClientTrait.php

Lines 746 to 748 in 9b06449

	foreach ($descriptor['additionalArgumentMethods'] as $additionalArgsMethodName) {
	$additionalArgs[] = $request->$additionalArgsMethodName();
	}

But since these need to be set on the Request object, we could pass $additionalArgs in with array keys and use those in OperationResponse to set the fields on the Request.

Example (keys as protobuf field names):

// in descriptor_config.php
        'additionalArgumentMethods' => [
            'project' => 'getProject',
            'zone' => 'getZone',
        ],

// in OperationResponse::operationsCall
new $requestClass(['name' => $this->getName()] + $this->additionalArgs);

Example (keys as message setters):

// in descriptor_config.php
        'additionalArgumentMethods' => [
            'setProject' => 'getProject',
            'setZone' => 'getZone',
        ],

// in OperationResponse::operationsCall
$request = $requestClass::build($this->getName());
foreach ($this->additionalArgs as $setter => $value) {
    $request->$setter($value);
}