feat: add custom token exchange support across all platforms

[sanchitmehtagit]

• 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

Description

Implements Custom Token Exchange (RFC 8693) feature for Auth0 Flutter SDK, enabling users to exchange external identity provider tokens for Auth0 credentials.

Dependencies Updated

Auth0 Android: 3.11.0 → 3.12.0 (adds organization parameter to customTokenExchange - Dec 2025)
Auth0.swift: 2.14.0 → 2.16.2 (adds organization parameter to customTokenExchange - Dec 2025)

Core Implementation

• Android: Added CustomTokenExchangeApiRequestHandler with organization support (requires Auth0 Android SDK 3.12.0+)
• iOS/macOS: Added AuthAPICustomTokenExchangeMethodHandler with organization support (requires Auth0.swift 2.16.0+)
• Web: Added JS interop for auth0-spa-js exchangeToken() method with organizationId support
• Platform Interface: Added AuthCustomTokenExchangeOptions and ExchangeTokenOptions classes with organization parameter

API Methods

// Mobile (Android/iOS)

  auth0.api.customTokenExchange(
    subjectToken: 'external-token',
    subjectTokenType: 'urn:ietf:params:oauth:token-type:jwt',
    audience: 'https://api.example.com',
    scopes: {'openid', 'profile', 'email'},
    organization: 'org_id', // Optional - requires updated native SDKs
    parameters: {'key': 'value'} // Optional
  )

// Web

  auth0Web.customTokenExchange(
    subjectToken: 'external-token',
    subjectTokenType: 'urn:ietf:params:oauth:token-type:jwt',
    organizationId: 'org_id', // Optional
    scopes: {'openid', 'profile', 'email'}
  )

📎 References

🎯 Testing

[sanchitmehtagit]

review examples.md file again

[sanchitmehtagit]

Merging this tested on macos , ios, android and web platform