[flutter_appauth_platform_interface] Add Prompt class to expose all prompt defined in OpenID Connect 1.0 specs (#618)

* Add `Prompt` class to expose all prompt defined in OpenID Connect 1.0 specs

* apply requested API doc changes

---------

Co-authored-by: Michael Bui <[email protected]>

Author: vmichalak

flutter_appauth/lib/flutter_appauth.dart

@@ -13,6 +13,8 @@ export 'package:flutter_appauth_platform_interface/flutter_appauth_platform_inte
         FlutterAppAuthUserCancelledException,
         FlutterAppAuthPlatformException,
         GrantType,
+        Prompt,
         TokenRequest,
         TokenResponse;
+
 export 'src/flutter_appauth.dart';

flutter_appauth_platform_interface/lib/flutter_appauth_platform_interface.dart

@@ -9,5 +9,6 @@ export 'src/errors.dart';
 export 'src/external_user_agent.dart';
 export 'src/flutter_appauth_platform.dart';
 export 'src/grant_type.dart';
+export 'src/prompt.dart';
 export 'src/token_request.dart';
 export 'src/token_response.dart';

flutter_appauth_platform_interface/lib/src/prompt.dart

@@ -0,0 +1,42 @@
+/// All spec-defined values for the OpenID Connect 1.0 `prompt` parameter.
+///
+/// See OpenID Connect Core 1.0, Section 3.1.2.1 for more information:
+/// https://openid.net/specs/openid-connect-core-1_0.html#rfc.section.3.1.2.1
+class Prompt {
+  /// The Authorization Server MUST NOT display any authentication or consent
+  /// user interface pages.
+  ///
+  /// An error is returned if an End-User is not already
+  /// authenticated or the Client does not have pre-configured consent for the
+  /// requested Claims or does not fulfill other conditions for processing the
+  /// request. The error code will typically be `login_required`,
+  /// `interaction_required`, or another code defined in
+  /// [OpenID Connect Core 1.0, Section 3.1.2.6](
+  /// https://openid.net/specs/openid-connect-core-1_0.html#rfc.section.3.1.2.6).
+  /// This can be used as a method to check for existing authentication and/or
+  /// consent.
+  static const String none = 'none';
+
+  /// The Authorization Server SHOULD prompt the End-User for re-authentication.
+  ///
+  /// If it cannot re-authenticate the End-User, it MUST return an error,
+  /// typically `login_required`.
+  static const String login = 'login';
+
+  /// The Authorization Server SHOULD prompt the End-User for consent before
+  /// returning information to the Client.
+  ///
+  /// If it cannot obtain consent, it MUST
+  /// return an error, typically `consent_required`.
+  static const String consent = 'consent';
+
+  /// The Authorization Server SHOULD prompt the End-User to select a user
+  /// account.
+  ///
+  /// This enables an End-User who has multiple accounts at the
+  /// Authorization Server to select amongst the multiple accounts that they
+  /// might have current sessions for. If it cannot obtain an account selection
+  /// choice made by the End-User, it MUST return an error, typically
+  /// `account_selection_required`.
+  static const String selectAccount = 'select_account';
+}