IamProviderInterface for IAM Mixins

[bshaffer]

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:

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;
    
}