protected web api with GetAadIssuerValidator

```
- [ ] bug report -> please search issues before submitting
- [ ] feature request
- [ x] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

I am using .NET 5.0 and trying to validate the issuer. Do we have any samples doing this? I have not figure out how to get the service provider (IServiceProvider) in order to call GetRequiredService,

```
MicrosoftIdentityIssuerValidatorFactory microsoftIdentityIssuerValidatorFactory =
**_serviceProvider**.GetRequiredService();

options.TokenValidationParameters.IssuerValidator = microsoftIdentityIssuerValidatorFactory.GetAadIssuerValidator(options.Authority).Validate;
```