feat: Support OAuth 2.0 Device Authorization Grant (RFC 8628) in Pinniped CLI

[yuito-it]

Summary

This PR introduces/enhances support for the OAuth 2.0 Device Authorization Grant (RFC 8628) in the Pinniped CLI.

Background / Details

• Enables seamless authentication in browserless / SSH environments where direct browser redirect (PKCE authorization code flow) is not feasible.
• Prompts the user with a verification URL and user code, polling the IdP token endpoint until authentication is completed on another device.
• Ensures compatibility with standard OIDC/OAuth2 IdPs supporting the device authorization flow.

Fixes

resolve #3225

Release note

Added support for OAuth 2.0 Device Authorization Grant (Device Code Flow) in Pinniped CLI, allowing authentication in headless and SSH environments via a secondary device.

[netlify]

✅ Deploy Preview for pinniped-dev canceled.

Name	Link
🔨 Latest commit	3eb07cf
🔍 Latest deploy log	https://app.netlify.com/projects/pinniped-dev/deploys/6a75b105f5ef4c0008b4731b