Provide way to have DI services participate in the logger configuration (broken with 2.0 changes)

[epignosisx]

Following the announcement about the breaking changes in logging for 2.0, I noticed that we were losing the ability to add logging providers in Startup.Configure. I posted the question in the discussion issue and it was later confirmed by @davidfowl.

Before I was able to configure serilog as such:

public void Configure(ILoggerFactory factory, IHttpContextAccessor httpContextAccessor)
{
    var logger = new Serilog.LoggerConfiguration()
        .WriteTo.RollingFile("log-{Date}.txt")
        .Enrich.With(new WebEnricher(httpContextAccessor))
        .CreateLogger();

    factory.AddSerilog(logger);
}

Note how the WebEnricher depends on the IHttpContextAccessor. With the new style of configuring logging, we do not have access to DI.

[davidfowl]

/cc @pakrym

[pakrym]

Fixed via #626
You can now inject services into ILoggerProvider