feat: Add telemetry support for flutter windows

[NandanPrabhu]

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

📋 Changes

Adds the Auth0-Client instrumentation/telemetry header to the Windows /oauth/token POST request

New — BuildAuth0ClientHeader(name, version) helper (auth0_client.h/cpp):

• Builds a base64url-encoded JSON payload sent as the Auth0-Client HTTP header:

  {"name":"auth0-flutter","version":"2.1.0","env":{"Windows":"10.0"}}

• Uses RFC 4648 §5 base64url encoding (URL-safe alphabet, no padding)
• Detects the real Windows OS version via RtlGetVersion (works correctly on Windows 10+ where GetVersionEx lies)

Modified — Auth0Client class (auth0_client.h/cpp):

• Constructor now accepts a third auth0ClientHeader string parameter
• ExchangeCodeForTokens attaches the Auth0-Client header on the token exchange request when the value is non-empty

Modified — LoginWebAuthRequestHandler (login_web_auth_request_handler.cpp):

• Extracts the _userAgent map (name + version) from Dart method call arguments
• Calls BuildAuth0ClientHeader(name, version) and passes the result into Auth0Client
• The Dart layer already sends _userAgent on every WebAuthRequest via the platform interface Request base class — no Dart changes needed

Platform consistency:

Platform	env field
iOS/macOS	{"iOS":"17.0","swift":"5.9"}
Android	{"android":"34"}
Windows	{"Windows":"10.0"}

All platforms use the same JSON schema and base64url encoding.

📎 References

• Auth0.swift telemetry implementation: Auth0/Telemetry.swift (sets Auth0-Client header on every request)
• Dart-side _userAgent source: auth0_flutter_platform_interface/lib/src/request/request.dart:16
• RFC 4648 §5 (base64url encoding): https://datatracker.ietf.org/doc/html/rfc4648#section-5

🎯 Testing

Automated (5 unit tests in auth0_client_telemetry_test.cpp):

• IsNotEmpty — header is always produced
• ContainsNoBase64Padding — no = padding characters
• ContainsNoStandardBase64Chars — + → -, / → _
• DecodestoValidJson — decoded payload contains expected name, version, env, and Windows keys
• EmptyNameAndVersionStillProducesValidHeader — graceful fallback when Dart doesn't supply agent info

Manual verification:

1. Run the example app on Windows, complete a login flow
2. Check Auth0 tenant logs to confirm the Auth0-Client header is present on the /oauth/token request
3. Decode the header value (base64url → JSON) and verify it contains the correct SDK name, version, and Windows OS version