feat: Cortex supports custom API Keys

[louis-jan]

Problem
I want to host an in-house AI agent and make it publicly accessible from anywhere, as I have some on-device applications that need to access LM endpoints to function properly.

The concern is that when exposing my self-hosted endpoint without a gate, even a simple one.

Success Criteria
cortex -h 0.0.0.0 -p 1102 -k [my_secret_goes_here]

Additional context
Add any other context or screenshots about the feature request here.

[dan-menlo]

Merging this into #551