feat: allow concurrent artifact uploads

[richardgetz]

Which issue(s) does this change fix?

• Feature request: Deploy Lambdas with concurrency #7056

Why is this change necessary?

• Deployments with many Lambda/image artifacts currently serialize all uploads, which can take minutes on larger
  stacks. Allowing users to opt into parallel uploads lets them shorten the pre-deploy packaging phase when
  bandwidth and CPU permit.

How does it address the issue?

• Adds a --parallel-upload flag (and guided/config plumbing) so sam deploy runs the packaging phase with concurrent
  S3/ECR uploads.
• Reworks Template.export to submit resource uploads to a ThreadPoolExecutor, sharing a thread-safe deduplication
  cache and guarding the ECR uploader’s login state.
• Propagates the flag through PackageContext/DeployContext, CLI telemetry output, and schema/config metadata.

What side effects does this change have?

• When enabled, artifact uploads will consume more CPU/network in exchange for faster completion. Default behavior
  remains the same (serial uploads) unless the flag/config setting is used.

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Review the generative AI contribution guidelines
• Add input/output type hints to new functions/methods
• [NA] Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• [NA] Write/update integration tests
• [NA] Write/update functional tests if needed
• make pr passes
• [NA] make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.