Overload Configure with access to IServiceProvider

[fredrikhr]

IServiceCollection support adding ServiceDescriptors with a factory func, rather than an actual instance. It would be helpful, if Configure() would provide an overload where the IConfiguration instance is obtained from the serviceProvider. Currently you cannot configure options when you do not have access to the IConfiguration instance when you call Configure(). This might be the case if you have the IConfiguration stored in the DI container.

[fredrikhr]

Proposed solution: #219

[tuespetre]

@couven92 with 2.0 at least, and using the WebHostBuilder, the ConfigureServices method of WebHostBuilder has an overload that can receive a WebHostBuilderContext that exposes the Configuration.

[shirhatti]

Implement IConfigureOptions instead of calling .Configure()
Your concrete implementation can depend on whatever services you need including IServiceProvider