Remove top level methods on IServiceCollection that configure authentication

[davidfowl]

services.AddScheme is too general of a name to put on the top level container:

Is it even meaningful to add a scheme without adding the authentication service? Why isn't it an extension method off on an IAuthenticationBuilder or something similar.

services.AddAuthentication()
              .AddScheme(...)

[HaoK]

Not a bad idea to switch to a builder pattern. Do we want to move everything under, i.e.?

services.AddAuthentication().AddScheme/Xyz()

[HaoK]

So something like:
Authentication.Core has void services.AddAuthenticationCore() // unchanged
Authentication adds AuthenticationBuilder services.AddAuthentication() builder.AddScheme
Authentication.Xyz adds AuthenticationBuilder.AddXyz()

[davidfowl]

I was also thinking we should hang the services like cookie, jwt, etc off the builder.

services.AddAuthentication()
              .AddCookies(...)
              .AddTwitter(...)

[HaoK]

Yup that's what I meant with the AddXyz.