Using Startup class not from the execution Assembly

### Describe the bug
Hey,

I am trying to use a Startup class like that
`WebHost.CreateDefaultBuilder(args).UseStartup().Build().Run()`
When the Startup class is located in the execution assembly everything works fine.
When the Startup class is located in a different assembly, the service runs but things go wrong during requests - the service throws a 404 without any explanation.

Why is it important for me to put my Startup class in a different assembly?
Because I have many services that are using the exact same Startup configuration and I want to prevent duplication.

### To Reproduce
Steps to reproduce the behavior:
1. Creating new template of Asp.net core 2.2 WebApi
2. Request is working
3. Creating new Project of .Net Standard 2.0
4. Moved the Startup class from the Asp.net Core project to the .Net Standard Project
5. Added these two nugets to the .Net Standard project : 'Microsoft.AspNetCore.Mvc' and 'Microsoft.AspNetCore.Diagnostics' Nugets.
6. Add Reference from the Asp.net core project the .Net Standard project
7. Request is not working

Thank you :)