[Feature]: Support headers for reverse proxy authentication (Identity Aware Proxies)

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Summary

Identity Aware Proxies (IAP) are used to implement the Zero Trust security model which uses the user identity instead of network-based as done by traditional VPNs.

These set specific headers, like X-USER-NAME, which is parsed by the backend to authenticate/create the user. For e.g. Gafana supports this: https://grafana.com/docs/grafana/latest/auth/auth-proxy/

### Why should this be worked on?

This feature would allow deploying appsmith in organizations that have implemented IAP solutions, where authentication happens in a single place.

[Front conversations](https://app.frontapp.com/open/top_2l3txo)