feat: add etl-engine AGENTS.md

Jean-Gabriel Doyon PTO until 2024-04-17requested to merge
add-etl-engine-agents.md into main

What does this MR do and why?

Adds an AGENTS.md file to the etl-engine crate and references it from the main repository AGENTS.md.

Changes

  • Created crates/etl-engine/AGENTS.md covering:
    • Architecture and components
    • Trait definitions (Handler, Module, Destination, Event)
    • Test utilities and workflow
    • Common tasks
    • Configuration reference
  • Added etl-engine to the Crates section in root `AGENTS.md

Related Issues

N/A

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