ci: Add workflow to publish built layers as artifacts

[CarlesDD]

What does this PR do?

This PR adds a GH workflow to build layers on each PR.

Motivation

The goal is to have artifacts for up-to-date development version of the layers for the system-tests to consume easily.

This allows the system-tests repo to just fetch a layer without needing any knowledge on how to build layers.

Testing Guidelines

The new workflow has been tested in this branch. You can find the execution here: https://github.com/DataDog/datadog-lambda-js/actions/runs/23647614074

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

APPSEC-61725