Add key file support with security hardening and HTTP redirects

[professor93]

Summary

• Add auth_jwt_key_file directive for loading JWK/JWKS key files
• Add auth_jwt_key_file_missing_skip to optionally bypass auth when key file is missing
• Add auth_jwt_key_file_missing_error for custom error responses or HTTP redirects
• Fix critical security and memory safety issues:
  • Null-termination bugs when passing ngx_str_t to C functions
  • Buffer overflow in base64url_decode for bytes >= 128
  • File descriptor leak in load_public_key
  • Missing NULL checks after ngx_palloc

New Directives

auth_jwt_key_file "/path/to/keys.jwks";
auth_jwt_key_file_missing_skip on|off;
auth_jwt_key_file_missing_error 503;
auth_jwt_key_file_missing_error 302 "https://login.example.com";