Fetch from S3 and manage run_instances_overrides.json on head node

[hanwen-cluster]

Support fetching run_instances_overrides.json from the cluster S3 bucket during head node bootstrap and update. This file applies customer-specified RunInstances overrides when launching compute instances.

Follows the instance_types_data pattern: S3 version-pinned fetch, backup to previous file on update for rollback support.

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[hanwen-cluster]

This PR is replaced by aws/aws-parallelcluster#7268