sample keycloak integration

[kstan79]

i think the provided example very bad for keycloak user, so I contribute some example of setting for others people to refer, I hope you guys can prepare better sample for community.

final String _clientId = 'keycloak-client-id';
  final String _redirectUrl = 'com.domain.appname:/oauthredirect';
  final String _issuer = 'https://login.your.url/realms/yourrealm';  // you may put `auth` behind hostname if your keycloak very old
  final String _discoveryUrl =
      'https://login.your.url/realms/yourrelam/.well-known/openid-configuration';
  final String _postLogoutRedirectUrl = 'com.domain.yourapp:/';

...
authorizationEndpoint:
        'https://login.your.url/realms/yourrealm/protocol/openid-connect/auth',
    tokenEndpoint:
        'https://login.your.url/realms/yourrealm/protocol/openid-connect/token',
    endSessionEndpoint:
        'https://login.your.url/realms/yourrealm/protocol/openid-connect/logout',
.....

final AuthorizationTokenResponse? result =
          await _appAuth.authorizeAndExchangeCode(
        AuthorizationTokenRequest(
          _clientId, _redirectUrl,
          serviceConfiguration: _serviceConfiguration,
          scopes: _scopes,
          preferEphemeralSession: preferEphemeralSession,
          clientSecret: '_your_client_secret_obtain_from_somewhere'  //original example not include this properties cause authorization never success
        ),
      );