[Platform] Support Google Vertext AI as a platform

[aszenz]

Allow using gemini models from google cloud, gemini models are accessible via Google Vertex AI which is a similar kind of service to AWS bedrock

https://cloud.google.com/vertex-ai

We could use the google cloud php ai sdk to implement it as it supports credential chain discovery, similar to async bedrock runtime client, see:

https://github.com/googleapis/google-cloud-php-ai-platform

Example rawPredict request:

$predictionServiceClient = new Google\Cloud\AIPlatform\V1\Client\PredictionServiceClient();
$modelUrl = \sprintf(
    'projects/%s/locations/%s/%s',
    $this->googleCloudProject,
    self::GOOGLE_CLOUD_LOCATION,
    $modelId,
);
$response = $predictionServiceClient->rawPredict(
    request: RawPredictRequest::build(
        endpoint: $modelUrl,
        httpBody: new HttpBody(
            [
                'data' => \json_encode($payload)
            ]
        )
    )
);

See preview relevant issue: #146

[chr-hertel]

Sounds good, PR welcome :)