[Feature Request] Change from delegate to interface for validators.

[brentschmaltz]

The current extensibility model is to use delegates:

azure-activedirectory-identitymodel-extensions-for-dotnet/src/Microsoft.IdentityModel.Tokens/Experimental/Delegates.cs

Line 21 in f58d585

public delegate ValidationResult<string, ValidationError> AlgorithmValidationDelegate(

This request is to move to interfaces.