title: IdentityServer 4 login with username or phone number
body: <p>In our product, we have mobile app interact with API which auth against  <code>IdentityServer 4</code>,
  the business set a requirement to be able to login using a phone(SMS auth) or username auth for the same person.
  I found this example project in order to login with phone SMS and it works great:
  <code>IdentityServer4.PhoneNumberAuth</code></p>
  
  <p>How can I combine a username login as well?</p>