Have ServiceEndpointResolver implement an interface

### Details of issue
In my aspnet core application I am utilising dependency injection to provide ServiceEndpointResolver in the constructor of my controllers which works as expected for my application. However now that I am reviewing my unit tests, all the controllers which utilise ServiceEndpointResolver as an argument are failing even after using Moq to create the parameter for the constructor.

using the mock repository results in the below error
` System.NotSupportedException: Type to Mock (ServiceEndpointResolver) must be an interface, a delegate or a non-sealed, non-static class.`

Attempting to use Nsubsitute results in the below error
` System.NotSupportedException: Parent does not have a default constructore. The default constructor must be explicitly defined.`

What i would like to make it easier for developers is if ServiceEndpointResolver was an implementation of IServiceEndpointResolver to allow for test cases to easily be implemented and pass.

### Workaround available
Implementation of a dedicated constructor without ServiceEndpointResolver and additional code in class to handle the null case.