feat(microvm): add image build example

[edersonbrilhante]

Description

Adds the repository-local Lambda MicroVM image-build flow and runner example.

• Adds the ARM64 Packer template, pinned Docker image definition, image builder, publisher, and runtime entrypoint.
• Packages the GitHub Actions runner, CloudWatch Agent, and lifecycle-hook server into the MicroVM image.
• Defines the external compiled lifecycle-hook ZIP contract, including root-level server.js validation.
• Adds the examples/microvm consumer and image-build workflow/documentation updates.

Test Plan

• Packer formatting and validation with representative non-secret values
• ShellCheck and Hadolint for image scripts
• Lifecycle-hook ZIP validation with root-level server.js
• terraform validate for examples/microvm
• git diff --check

Related Issues

Depends on #5377.

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None