Service Fabric Autofac example for AspNet DotNetCore 3.1+

## No examples on how to use Autofac in AspNet Core (DotNet Core 3.1+) web applications that run in Service Fabric

The repo at https://github.com/autofac/Examples/tree/master/src/ServiceFabricDemo/AutofacServiceFabricDemo hasn't been updated in 3 years. A lot has changed since then.

For instance, `Startup.cs` must have a void `ConfigureServices(IServiceCollection services)` method. Returning `IServiceProvider` is not valid anymore after DotNetCore 3.1+

I have created the following branch with a web app running the latest DotNetCore, that uses service fabric and Microsoft Dependency Injection. [See here](https://github.com/diegosasw/issue-aspnetcore-servicefabric-autofac-testserver/tree/feature/microsoft_dependency_injection)

It fulfills all the requirements (DI, reading appsettings, clean change of environment for testing, TestServer with possibility to overwrite DI registrations for mocks, etc.). Please feel free to have a look and if there's any way to add Autofac to that sample I'll be happy to contribute.

There are unanswered questions in StackOverflow about the same:
https://stackoverflow.com/questions/59980827/service-fabric-aspnet-core-3-1-autofac-webhostbuilder
https://stackoverflow.com/questions/64260534/how-to-use-service-fabric-service-with-aspnet-core-webapi-and-autofac-and-run-te
https://stackoverflow.com/questions/64278765/how-to-use-autofac-with-iwebhost

## Desired Solution
A basic example on how to register Autofac with a `WebHostBuilder` (if possible), and where to register dependencies in a service fabric app would be enough.

## Alternatives You've Considered

The alternative I've considered is not to use Autofac, but if I could help in any way to find out how to do it or to suggest something, I'll do that.