Add built in support for validating options about options

[MaKCbIMKo]

I think this is something similar with what you're looking for.

Am I right?

[HaoK]

This feature is on our radar, I'm not sure when we will add first class support for validation of options, (today we roll our own validation pattern in Auth), but sooner or later this seems like something Options should support in a first class way.

cc @divega @ajcvickers @davidfowl

[MaKCbIMKo]

Some time ago I was thinking about being able to validate options. In my opinion, this is really useful feature - being able to validate options and handle invalid scenarios.

Moreover, I was looking for possibility to validate all registered options at start-up time (not only when it will be required) - this is just a simple early problem detection.

For that purpose I have created PR (#171) which adds the possibility to configure the validation of option that's going after all configure steps. And that PR introduces the possibility to validate all options at the start up time.

[aspnet-hello]

This issue was moved to dotnet/aspnetcore#2388