feat: add mTLS support via clientCertSource client option

[bshaffer]

• Adds client option clientCertSource
• Adds support for client certs to all three transports
• Verified calls to mTLS endpoint works locally

Usage

use Google\Cloud\PubSub\V1\PublisherClient;

$projectId = 'YOUR_PROJECT_ID';
$pubsub = new PublisherClient([
    'apiEndpoint' => 'pubsub.mtls.googleapis.com',
    'clientCertSource' => function () { 
         return file_get_contents(__DIR__ . '/cert.pem');
    },
]);

$topics = $pubsub->listTopics('projects/' . $projectId);

TODO

• Add support for checking the existence and loading the default client cert (from ~/.secureConnect/context_aware_metadata.json) (done in feat: add loading and executing of default client cert source google-auth-library-php#353)
• Add support for GOOGLE_API_USE_MTLS_ENDPOINT and GOOGLE_API_USE_CLIENT_CERTIFICATE
• Add support to automatically use mTLS endpoint for apiEndpoint
• Tests