title: What is the difference between AddAuthentication and AddAuthenticationCore?
body: <p>Looking at the code for <a href="https://github.com/dotnet/aspnetcore/blob/62351067ff4c1401556725b401478e648b66acdc/src/Security/Authentication/Core/src/AuthenticationServiceCollectionExtensions.cs" rel="noreferrer">AuthenticationServiceCollectionExtensions.AddAuthentication()</a> vs <a href="https://github.com/dotnet/aspnetcore/blob/425c196cba530b161b120a57af8f1dd513b96f67/src/Http/Authentication.Core/src/AuthenticationCoreServiceCollectionExtensions.cs" rel="noreferrer">AuthenticationCoreServiceCollectionExtensions.AddAuthenticationCore()</a>, it looks like AddAuthentication implicitly calls AddAuthenticationCore, adds some other goodies, and then returns a new instance of AuthenticationBuilder instead of just returning IServiceCollection.</p>
  
  <p>Am I understanding the code correctly?  If so, are there generally any reasons to call AddAuthenticationCore instead of AddAuthentication outside of writing your own extension?</p>