Support configuration "chaining", or adding one configuration to another

[DamianEdwards]

Required to support aspnet/Hosting#448

Add support for adding one IConfiguration to another, e.g.:

public Startup(IHostingEnvironment hosting)
{
    var builder = new ConfigurationBuilder()
        .AddJsonFile("appsettings.json")
        .AddEnvironmentVariables()
        .AddConfiguration(hosting.Configuration);

    Configuration = builder.Build();
}

Should have two extension method overloads:

public static IConfigurationBuilder AddConfiguration(this IConfigurationBuilder builder, IConfiguration configuration);
public static IConfigurationBuilder AddConfiguration(this IConfigurationBuilder builder, string prefix, IConfiguration configuration);

[davidfowl]

/cc @divega

[divega]

Cool. A ConfigurationConfigurationProvider 😏 Is this needed in RC2?

[davidfowl]

Yes

[DamianEdwards]

@divega can you assign please?

[divega]

Figuring out with @Eilon who will work on this.

[divega]

Assigning to @HaoK. Hopefully he can start in a few days.

[HaoK]

2909753