Add LaunchTemplateOverrides support for compute resources

[hanwen-cluster]

Description of changes

Allow customers to specify LaunchTemplateOverrides with a LaunchTemplateId and Version at the compute resource level. This enables customers to customize RunInstances parameters (e.g. NetworkInterfaces) that are not directly exposed by ParallelCluster config, without requiring changes to the ParallelCluster codebase for each new parameter.

The launch template data is fetched, transformed into run_instances_overrides.json (keyed by queue/compute resource name), and uploaded to the cluster S3 bucket. The file is always uploaded (empty {} when no overrides) to cleanly handle cluster update transitions.

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