roadmap: Engines are `dylibs` and are self-contained

[freelerobot]

Goal

• Engines are modular and 3rd parties can add engines
• Engine loading needs to be moved to engine repo (e.g. llamacpp-engine)
• Engines can be registered dynamically, declaratively (e.g. .cortexrc)
• Engines have clear and consistent interface

Tasklist

• Refactor Engine Loading out of Cortex to engine extensions
• .cortexrc can define engines
• docs: how to build an Engine #1731
  • How to add a third party engine to Cortex (and Jan - optionally)
  • How to register an new engine for official distribution

PRs

• fix: load engine linux #1790