Feature/api key config

[DevastatingRPG]

Description

Added support for passing an API key via the config for the LiteLLM provider. This involved a small internal update to accept the key and a documentation update to reflect this option.

Fixes #1746

Type of change

Please delete options that are not relevant.

• [✔️ ] New feature (non-breaking change which adds functionality)
• [ ✔️] Documentation update

How Has This Been Tested?

• Ran the existing unit tests with no conflicts
• Manually tested by passing dummy API key via the config and verified that it was correctly picked up by litellm provider without needing to set environment variables

No new test cases were added as no valid API key was available for OpenAI to mock the requests without errors.

Checklist:

• [✔️ ] My code follows the style guidelines of this project
• [ ✔️] I have performed a self-review of my own code
• [ ✔️] I have commented my code, particularly in hard-to-understand areas
• [✔️ ] I have made corresponding changes to the documentation
• [✔️ ] My changes generate no new warnings
• [❌ ] I have added tests that prove my fix is effective or that my feature works
• [✔️ ] New and existing unit tests pass locally with my changes
• [✔️ ] Any dependent changes have been merged and published in downstream modules
• [✔️ ] I have checked my code and corrected any misspellings

Maintainer Checklist

• [✔️ ] closes #1746
• [✔️ ] Made sure Checks passed

[CLAassistant]

All committers have signed the CLA.