title: How to add OnTokenValidated event handler when using AD B2C?
body: <p>I am using Azure B2C in a ASP.NET Core 3 application, which is working perfectly.
  I use the following code in Startup: </p>
  
  <pre><code>services.AddAuthentication(AzureADB2CDefaults.AuthenticationScheme)
      .AddAzureADB2C(options =&gt; Configuration.Bind("AzureAdB2C", options));
  </code></pre>
  
  <p>I would like to handle the standard TokenValidated OpenIdConnect event, with other words I need a configuration where my event handler is set.</p>
  
  <p>Examining the source code I see the class <code>AzureAdB2COpenIDConnectEventHandlers.cs</code> and also its usage in <code>AzureADB2COpenIdConnectOptionsConfiguration</code> but unfortunately both class declared to <code>internal</code></p>
  
  <p><strong>Question</strong></p>
  
  <p>All I need is to have my TokenValidated handler in effect, retaining all working out of the box OpenIdConnect based AD B2C functionality, which is working currently.</p>
  
  <p>Pseudo code, something like this: </p>
  
  <pre><code>options.Events = new OpenIdConnectEvents()
  {
       // ...
       OnTokenValidated = MyTokenValidatedHandler
  };
  </code></pre>
  
  <p>How can I accomplish this in a simple way?</p>