feat: cortex extensions

[louis-jan]

Description

We will introduce Extensions as part of the Cortex Backend, allowing us to migrate our current remote inference engine extensions into Cortex.

Some questions about how it should work alongside Cortex-JS:

1. Where are these extensions located? Should the repo be separated or within the cortex-js folder?
2. How they can be activated or deactivated?
3. Would it be scoped only by the Inference Engine?
4. How is it designed within a NestJS backend?

graph RL
Cortex[?cortex-cpp Engine Extension] -->|inject| Backend
OpenAI[?Mistral Engine Extension] --> |inject| Backend
Cohere[?OpenAI Engine Extension] --> |inject| Backend
Groq[?Groq Engine Extension] --> |inject| Backend
... --> |inject| Backend

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Subtask

• Headless Backend Extensions jan#2782
• Refactor Inference Engine extensions to Backend jan#2783