Introduce SEND_USERNAME_TO_MODEL to send username instead of id to model calls

[alihacks]

Summary

Some users may benefit from logging username instead of user id when calling LLMs, this change introduces environment variable SEND_USERNAME_TO_MODEL which when set to true will send username instead of id.

Change Type

Please delete any irrelevant options.

• New feature (non-breaking change which adds functionality)

Testing

Tested with Azure Open AI and confirmed username can be passed when this variable is set

Checklist

Please delete any irrelevant options.

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• I have made pertinent documentation changes
• My changes do not introduce new warnings
• Local unit tests pass with my changes
• A pull request for updating the documentation has been submitted.

[alihacks]

Sorry -I have been away last week. Doc updates are ready at LibreChat-AI/librechat.ai#149