docs: update sam example to nodejs18

[dreamorosi]

Description of your changes

As detailed in #1195, the SAM Examples in the repo needed some changes to start using Node.js 18.

This PR builds upon the changes #1180 and:

• Sets functions runtime to nodejs18x
• Updates bundling configs to exclude @aws-sdk/* packages (they're now part of the runtime)
• Moves functions code back to the folder (symlinks were not working well with jest)
• Light updates to function's code & dependencies
• Light updates to README file

How to verify this change

See unit tests in the PR that prove that the examples build successfully.

I have also manually deployed & tested the sample in an AWS account and verified that they work as intended.

Related issues, RFCs

Issue number: #1195

PR status

Is this ready for review?: YES
Is it a breaking change?: NO

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding changes to the examples
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published
• The PR title follows the conventional commit semantics

Breaking change checklist

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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