Add OAuth2Proxy (header-based) authentication

Add OAuth2Proxy (header-based) authentication: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). We deploy Keep behind oauth2-proxy (https://github.com/oauth2-proxy/oauth2-proxy), which terminates authentication for us and forwards the authenticated identity to the…

The task

We deploy Keep behind [oauth2-proxy](https://github.com/oauth2-proxy/oauth2-proxy), which terminates authentication for us and forwards the authenticated identity to the backend as HTTP headers. We want Keep to trust those headers as an authentication method, so no Keep-managed login/JWT is involved.

Part of FineEnvs/MiMo-V2.6-RL-harbor-code.