Enable proxy authentication

[trippgrace]

We host MB behind Google IAP, so HTTP requests need to have a proxy auth bearer token. This allows us to handle login in our environment context

• from Feat: Allow specifying PROXY_AUTH values via env-var (allows IAP) CognitionAI/metabase-mcp-server#19 , a PR in another Metabase MCP codebase. My org has the same configuration blocking Metabase MCP usage until we can authenticate.