Support authentication headers

# Summary

Support `X-Forwarded-User`/`X-Forwarded-Group` (or similar) headers for user authentication.

## Use Cases

We would like individual user authentication for Argo Workflows. However, we already have a standard identity proxy that we deploy in front of our internal services, that injects `X-Forwarded-User` headers.

Instead of configuring Argo do do its own authentication, we would like to re-use the already existing headers.

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Start by locating Argo Workflows' authentication configuration and HTTP request handling, then inspect existing authentication tests. The issue names no files or tests; completion should establish how X-Forwarded-User and X-Forwarded-Group are configured and trusted for individual-user authentication.

Written by the indexing model from the issue text.