docs: update CDK example to nodejs18

[dreamorosi]

Description of your changes

Following the merge of #1192, this PR introduces some changes to the CDK examples that are now updated to use the nodejs18x runtime.

The PR also includes other changes that, among other things, partially address the discussion in #520:

• Removed redundant functions
• Improved project structure with dedicated functions folder & shared code in functions/common
• Added API Gateway to align with the SAM
• Improved README file
• Removed auto-invoke logic in favor of user/reader invoking functions via API Gw
• Moved all functions to use Lambda Layers

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: #520

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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