Migrate the Timeout Policy

[martintmk]

Migrate the Timeout Policy to V8 API.

Prerequisites
#1070
#1067

The details on how to introduce the new resilience strategies for V8 are here.

The steps in general:

• Move exceptions and enums and classes that are shared between V7 and V8 into Polly.Core
• Use Type Forwarding so the consumers still using V7 won't get runtime errors.
• Introduce TimeoutOptions
• Expose new extensions for the timeout strategy.
• Forward the V7 implementation to V8.

API

public class TimeoutStrategyOptions
{
   public TimeoutStrategy Strategy { get; set; }

   public Event<OnTimeoutArguments> OnTimeout { get; set; }

   public Generator<TimeoutGeneratorArguments, TimeSpan> TimeoutGenerator { get; set; }
}

public readonly struct OnTimeoutArguments
{
   public ResilienceContext ResilienceContext { get; }
   public TimeSpan ExecutionTime { get; }
}

public readonly struct TimeoutGeneratorArguments
{
   public ResilienceContext ResilienceContext { get; }
}

public static class TimeoutResilienceStrategyBuilderExtensions
{
   ResilienceStrategyBuilder AddTimeout(this ResilienceStrategyBuilder builder, TimeoutStrategyOptions options);
   ResilienceStrategyBuilder AddTimeout(this ResilienceStrategyBuilder builder, TimeSpan timeout);
   ResilienceStrategyBuilder AddTimeout(this ResilienceStrategyBuilder builder, TimeSpan timeout, TimeoutStrategy  strategy);

   // possibly more
}

API Usage

// advanced setup
var options = new TimeoutStrategyOptions();

// add an event that's called when timeout occurrs
options.OnTimeout.Add(args => Console.WriteLine("Timeout occurred!"))

// add callback to result generator, you can return different timeouts based on the result type or data from resilience context
options.TimeoutGenerator.Add(args => TimeSpan.FromSeconds(5));
var strategy = new ResilienceStrategyBuilder().AddTimeout(options).Build();

// simple setup
var strategy = new ResilienceStrategyBuilder().AddTimeout(TimeSpan.FromSeconds(5)).Build();