WebHost service provider fails to handle covariant types

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#### Investigative information

Please provide the following:

- Timestamp: N/A
- Function App version: N/A
- Function App name: N/A
- Function name(s) (as appropriate): N/A
- Invocation ID: N/A
- Region: N/A

#### Repro steps

1. Create a new AzF in Visual Studio (any trigger).
2. Create a Startup class with the associated `FunctionsStartup` attribute.
3. Add a service registration for a concrete implementation of a covariant interface.
4. Start the function in the emulator.

#### Expected behavior

The service should start without errors.

#### Actual behavior

An exception is logged in the console:
> A host error has occurred during startup operation
> Microsoft.Azure.WebJobs.Script.WebHost: Registering implementation type IFactory is not assignable to service type Factory.

#### Known workarounds

No known workarounds for this problem; covariant type implementations are considered by the host to be unassignable to the interface.

#### Related information

A simple example of this problem:

```csharp
using Microsoft.Azure.Functions.Extensions.DependencyInjection;
using Microsoft.Extensions.DependencyInjection;
using System;

[assembly: FunctionsStartup(typeof(CovarianceTest.Startup))]

namespace CovarianceTest
{
public interface IFactory
{
T Create();
}

public class Factory : IFactory
{
public T Create()
{
throw new NotImplementedException();
}
}

public interface IClient
{
}

public class Client : IClient
{
}

public class Startup : FunctionsStartup
{
public override void Configure(IFunctionsHostBuilder builder)
{
builder.Services.AddTransient, Factory>();
}
}
}

Start with the repro's FunctionsStartup.Configure entry point and the builder.Services.AddTransient registration, then run the function in the emulator to observe the host validation error. Trace where the host checks assignability for the covariant IFactory and Factory types; done means the function starts without the exception.

Written by the indexing model from the issue text.