Add support for TooManyRequests 429 Polly policy

[kdcllc]

private IAsyncPolicy<HttpResponseMessage> GetRetryPolicy(int numRetries = 10, int delayInMilliseconds = 10000)
{
	var retryPolicy = Policy
		.Handle<HttpRequestException>()
		.OrResult<HttpResponseMessage>(r => r.StatusCode == HttpStatusCode.TooManyRequests ||
											r.StatusCode == HttpStatusCode.ServiceUnavailable)
		.WaitAndRetryAsync(numRetries,
			sleepDurationProvider: (retryCount, response, context) =>
			{
				var delay = TimeSpan.FromSeconds(0);

				// if an exception was thrown, this will be null
				if (response.Result != null)
				{
					if (!response.Result.Headers.TryGetValues("Retry-After", out IEnumerable<string> values))
						return delay;

					if (int.TryParse(values.First(), out int delayInSeconds))
						delay = TimeSpan.FromSeconds(delayInSeconds);
				}
				else
				{
					var exponentialBackoff = Math.Pow(2, retryCount);
					var delayInSeconds = exponentialBackoff * delayInMilliseconds;
					delay = TimeSpan.FromMilliseconds(delayInSeconds);
				}

				return delay;
			},
			onRetryAsync: async (response, timespan, retryCount, context) =>
			{
				// add your logging and what you want to do
			}
		);

	return retryPolicy;
}

App-vNext/Polly.Extensions.Http#26