Preview: Future reservation requests in calendar mode support reserving

## Feature Feature **Preview**: Future reservation requests in calendar mode support reserving capacity for `a3-megagpu-8g` and `a3-highgpu-8g` machine types. Use future reservation requests in calendar mode to obtain resources for running model pre-training, model fine-tuning, simulation, and inference workloads. For more information, see[About future reservation requests in calendar mode](https://cloud.google.com/compute/docs/instances/future-reservations-calendar-mode-overview).