Add call_activity_with_retry to DurableOrchestrationContext class

💡 **Feature description**
The `DurableOrchestrationContext` class has both a `call_activity_with_retry` method and a `call_sub_orchestrator_with_retry` method. Often times the call_http method fails for unknown and external reasons. Adding a `call_http` method that includes an option to retry a specified number of times would go a long way to improve the reliability of durable orchestrations. Adding an exponential backoff would be a nice plus.

💭 **Describe alternatives you've considered**
The obvious alternative is to wrap your API call into an activity function and use the `call_activity_with_retry` method. Creating new activity functions for each API call tends to bloat projects though. The v2 programming model may address this concern though - I'm still stuck on the v1 programming model for now.

まず DurableOrchestrationContext クラスと、既存の call_activity_with_retry メソッドおよび call_sub_orchestrator_with_retry メソッドを調べます。それらの動作を call_http と比較し、指定された再試行回数が何を意味するべきかを定義します。完了時には HTTP 呼び出しの再試行を提供し、指数バックオフはオプションの拡張とします。

索引モデルが issue の本文から書いたものです。