IamProviderInterface for IAM Mixins

It would be nice to typehint V2 GAPIC clients which implement IAM methods - For instance, we could have an `IamProviderInterface` in GAX which ensured that the methods being called on this GAPIC client exist. The generator could add this when it detects an IAM-mixin:

```php
namespace Google\ApiCore;

use Google\Cloud\Iam\V1\GetIamPolicyRequest;
use Google\Cloud\Iam\V1\Policy;
use Google\Cloud\Iam\V1\SetIamPolicyRequest;
use Google\Cloud\Iam\V1\TestIamPermissionsRequest;
use Google\Cloud\Iam\V1\TestIamPermissionsResponse;

interface IamProviderInterface
{
public function getIamPolicy(GetIamPolicyRequest $request, array $callOptions = []): Policy;
public function setIamPolicy(SetIamPolicyRequest $request, array $callOptions = []): Policy;
public function testIamPermissions(TestIamPermissionsRequest $request, array $callOptions = []): TestIamPermissionsResponse;

}