Introduce Polly.Extensions project

[martintmk]

The Polly.Extensions projects integrates the Polly with the standard IServiceCollection DI container and implements the ResilienceTelemetryFactory thus enabling the telemetry for all resilience strategies created using the DI extension points.

Example:

var services = new ServiceCollection();

// define your strategy
services.AddResilienceStrategy(
  "my-key", 
   context => context.Builder.AddTimeout(TimeSpan.FromSeconds(10)));

// define your strategy using custom options
services.AddResilienceStrategy(
    "my-timeout",
    context =>
    {
        var myOptions = context.ServiceProvider.GetRequiredService<IOptions<MyTimeoutOptions>>().Value;
        context.Builder.AddTimeout(myOptions.Timeout);
    });

// use your strategy
var serviceProvider = services.BuildServiceProvider();
var strategyProvider = serviceProvider.GetRequiredService<ResilienceStrategyProvider<string>>();

var resilienceStrategy = strategyProvider.Get("my-key");

// use your strategy

The consumer just calls the AddResilienceStrategy and provides a callback that configures the resilience strategy.

Optionally, the project can be also named Polly.DependencyInjection, however, the implementation of ResilienceTelemetryFactory shouldn't really belong there in that case.