* Consolidate authentication logic
- Moving auth functions from client/internal to client/internal/auth package
- Creating unified auth.Auth client with NewAuth() constructor
- Replacing direct auth function calls with auth client methods
- Refactoring device flow and PKCE flow implementations
- Updating iOS/Android/server code to use new auth client API
* Refactor PKCE auth and login methods
- Remove unnecessary internal package reference in PKCE flow test
- Adjust context assignment placement in iOS and Android login methods
Enhance the user experience by enabling authentication to Netbird using Single Sign-On (SSO) with any Identity Provider (IDP) provider. Current client offers this capability through the Device Authorization Flow, however, is not widely supported by many IDPs, and even some that do support it do not provide a complete verification URL.
To address these challenges, this pull request enable Authorization Code Flow with Proof Key for Code Exchange (PKCE) for client logins, which is a more widely adopted and secure approach to facilitate SSO with various IDP providers.
