Revisit SchemeBuilder/AddScheme

[HaoK]

We probably need some more sugar around adding schemes today, namely:

authOptions.AddScheme<handlerType>(schemeName, displayName)

which today looks like:

authOptions.AddScheme(schemeName, o => {
   o.DisplayName = displayName;
   o.HandlerType = typeof<HandlerType>;
});

In fact, now that we don't have settings in the scheme anymore, we can probably just remove the scheme builder and only have this api.

Thoughts @Tratcher ?

[Tratcher]

The current one is more future proof, but adding the sugar on top wouldn't hurt.

[HaoK]

Yeah I guess knowing how auth usually goes, the ability to add stuff later will probably be nice :)

[HaoK]

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