Make IConfigurationRefresher instance available via DI

[zhenlan]

It's useful for user code to obtain the instance of IConfigurationRefresher so they can refresh configuration synchronously or write their own middleware etc.

Proposal

Add extension method below that user can call in ConfigureServices(...)

public static IServiceCollection AddAzureAppConfiguration(this IServiceCollection services)

Under the cover, this API will add IConfigurationRefresherAccessor to the DI.

public interface IConfigurationRefresherAccessor
{
    IEnumerable<IConfigurationRefresher> Refreshers { get; set; }
}

User code can then get the instances of IConfigurationRefresher (via IConfigurationRefresherAccessor) from the DI.

Note: the implementation of IConfigurationRefresherAccessor will need to get IConfiguration from DI and cast it to IConfigurationRoot etc, just like what we do in the middleware today.

Other Considerations

• We may need to consider exposing other properties in IConfigurationRefresher so users can differentiate instances from one to another.
• We may consider changing our middleware to obtain instances of IConfigurationRefresher in the same way. This means users must call AddAzureAppConfiguration before they can call UseAzureAppConfiguration.

cc: @abhilasharora @jimmyca15 @drago-draganov

[drago-draganov]

I don't think there is a need for IConfigurationRefresherAccessor. DI can inject IEnunmerable<IConfigurationRefresher> directly.

[zhenlan]

We need to run the code that we have in the middleware today to get IEnunmerable<IConfigurationRefresher>, which is the reason I think we need IConfigurationRefresherAccessor. I will be glad if we don't have to. I will leave this to @abhilasharora to find out.

[drago-draganov]

It's a standard mechanism to add multiple instances of the same type T. Then the DI makes it available by requesting IEnumerable<T>

[abhilasharora]

I already have PR #78 out for changes that provide this functionality. I sent it last year, but it seems for some reason we decided to put it on hold.

[abhilasharora]

These changes are now available in version 4.0.0-preview of the package.