feat: additional nodepool resource with flex-start type support

[Ujjwal048]

Note

Medium Risk
Changes GKE node pool provisioning and compute/network tagging on live clusters; misconfiguration could affect capacity, scheduling (taints/GPU), or firewall rules tied to tags.

Overview
Adds a new additional_node_pools input (map, default {}) and a google-beta google_container_node_pool.additional_pools resource so callers can declare extra GKE node pools beyond the built-in control-plane and critical pools. Pools are only created when use_existing_cluster is false.

Each pool entry can tune machine/disk settings, autoscaling vs fixed node_count, taints/labels, optional GPUs (including sharing config), max_pods_per_node, node_locations, queued provisioning, and reservation affinity. Flex-start pools set flex_start, default max_run_duration to 7 days when omitted, force NO_RESERVATION, and skip preemptible/spot.

Locals merge per-pool resource_labels and network_tags (with module network_tags) for node metadata and firewall tagging. control_plane_pool_config and critical_pool_config gain optional network_tags, appended to their existing network tag lists.

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