Update JWT sample code for subscriptions

[Shane32]

The previous sample handler decoded the parsed JWT token directly using JwtSecurityTokenHandler. This works fine when the TokenValidationParameters were all configured (including the security keys). However, when using OIDC, the TokenValidationParameters should be generated from the ConfigurationManager, as it needs to download the keys from the OIDC endpoint before the TokenValidationParameter instance contains the keys. The revised code now is a copy (without much of the event and error handling) of the JwtBearerHandler logic, so that validation will work similarly to however it is configured within ASP.NET Core. Events are still not implemented at this time.

Keep in mind that while this code can be used as a guide for to how to write authorization logic for subscriptions, it is the user's responsibility to ensure that their endpoint is secure. Perhaps in the future we may publish this code in a NuGet package, but for now it is still sample code.