feat: add subnet_ids for multi-AZ spot instance failover

[jdrowne]

Description

Add a new subnet_ids parameter that allows users to specify multiple subnets for spot instances. When used with spot_instance_types, EC2 Fleet receives overrides for all instance type × subnet combinations, enabling automatic failover when capacity is constrained.

Example configuration:

source "amazon-ebs" "example" {
  spot_price          = "auto"
  spot_instance_types = ["m6i.large", "m5.large", "m6a.large"]
  subnet_ids          = ["subnet-abc123", "subnet-def456", "subnet-ghi789"]
}

This gives EC2 Fleet 9 placement options (3 instance types × 3 subnets) to choose from.

How it works:

• When subnet_ids is set, the launch template omits Placement/subnet
• Fleet overrides include the Cartesian product of instance types × subnets
• EC2 Fleet selects an available combination at launch time
• Existing subnet_id and subnet_filter behavior is unchanged

Resolved Issues

Closes #425

Rollback Plan

This change is additive and backward compatible. To rollback, revert the commit. Existing configurations using subnet_id or subnet_filter are unaffected.

Changes to Security Controls

No changes to security controls (access controls, encryption, logging).

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