Add IServiceScope support

[georg-jung]

This adds support for scoped services from the service provider using IServiceScope without taking a dependency on Microsoft.Extensions.DependencyInjection.Abstractions.

Motivation

I want to implement an IUserAuthenticator that consumes a UserManager<AppUser> userManager from ASP.NET Core Identity. This is typically registered as a scoped service. ISessionContext.ServiceProvider does however return the root SP. I think SMTP is a rather good fit for the scoped scenario. As a scope in Http corresponds to one request, a scope in Smtp can correspond to one Smtp session (one could argue it could also correspond to one SmtpCommand but I think the connection makes more sense for SMTP, given it is stateful).

Usage

If this is merged, one can easily add a class like this in his codebase that does depend on Microsoft.Extensions.DependencyInjection.Abstractions:

using SmtpServer;

namespace SomeApp;

internal class ScopedSessionServiceProviderFactory : ISessionServiceProviderFactory
{
    public IServiceProvider CreateServiceProvider(IServiceProvider rootServiceProvider)
    {
        return new DelegatingScopeProvider(rootServiceProvider.CreateAsyncScope());
    }

    // inspired by https://github.com/dotnet/runtime/blob/08298b48fd98df648d90c73e058522e4ac2eb008/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/AsyncServiceScope.cs
    private class DelegatingScopeProvider(IServiceScope delegateTo) : IServiceProvider, IAsyncDisposable, IDisposable
    {
        public void Dispose()
        {
            delegateTo.Dispose();
        }

        public ValueTask DisposeAsync()
        {
            if (delegateTo is IAsyncDisposable ad)
            {
                return ad.DisposeAsync();
            }

            delegateTo.Dispose();
            return ValueTask.CompletedTask;
        }

        object? IServiceProvider.GetService(Type serviceType) => delegateTo.ServiceProvider.GetService(serviceType);
    }
}

And register this class in the app's DI container like this:

builder.Services.AddSingleton<ISessionServiceProviderFactory, ScopedSessionServiceProviderFactory>();