Add IServiceProvider as an input to AddAuth0AuthenticationClient

[JasonWeinzierl]

It would be nice to have IServiceProvider available to fetch other services for populating configuration.

services.AddAuth0AuthenticationClient((config, provider) => {
    var configuration = provider.GetRequiredService<IConfiguration>();

    config.Domain = configuration["AUTH0_DOMAIN"];
    // etc.
});

[Hawxy]

Adding the overload isn't a big deal so I'll throw it into the next version, but IConfiguration is already readily available via *ApplicationBuilder/Startup.cs, which would be accessible to wherever this is being called.