Function launch_instances 

pub async fn launch_instances(
    client: &Ec2Client,
    ami_id: &str,
    instance_type: InstanceType,
    storage_size: i32,
    storage_class: VolumeType,
    key_name: &str,
    subnets: &[(String, String)],
    az_support: &BTreeMap<String, BTreeSet<String>>,
    start_idx: usize,
    sg_id: &str,
    count: i32,
    name: &str,
    tag: &str,
) -> Result<(Vec<String>, String), Error>

Launches EC2 instances with specified configurations. Filters subnets to those supporting the instance type, distributes across them starting at start_idx, and falls back to other subnets on capacity errors.