Add a virtual ShouldRetry method to the RetryPolicy for customization.

[ahsonkhan]

Fixes #5515

Related discussion:
#5530 (comment)

Given this is not meant for end customers to modify, I did not add it to the changelog to avoid highlighting it.

cc @Jinming-Hu

Here is how I would expect the storage use case to be implemented. The StorageRetryPolicy would inherit from RetryPolicy, and override the ShouldRetry method:

  bool ShouldRetry(std::unique_ptr<RawResponse> const& response, RetryOptions const& retryOptions)
      const override
  {
    if (response == nullptr)
    {
      return true;
    }

    if (static_cast<std::underlying_type_t<Azure::Core::Http::HttpStatusCode>>(
            response->GetStatusCode())
        >= 400)
    {
      const auto& headers = response->GetHeaders();
      auto ite = headers.find("x-ms-copy-source-status-code");
      if (ite != headers.end())
      {
        const auto innerStatusCodeInt = std::stoi(ite->second);
        const auto innerStatusCode
            = static_cast<Azure::Core::Http::HttpStatusCode>(innerStatusCodeInt);

        const bool shouldRetry
            = retryOptions.StatusCodes.find(innerStatusCode) != retryOptions.StatusCodes.end();

        if (Azure::Core::Diagnostics::_internal::Log::ShouldWrite(
                Azure::Core::Diagnostics::Logger::Level::Informational))
        {
          Azure::Core::Diagnostics::_internal::Log::Write(
              Azure::Core::Diagnostics::Logger::Level::Informational,
              std::string("x-ms-copy-source-status-code ") + std::to_string(innerStatusCodeInt)
                  + (shouldRetry ? " will be retried" : " won't be retried"));
        }
        if (shouldRetry)
        {
          return true;
        }
      }
    }
    return false;
  }

Follow-up improvement to remove such tight coupling in existing tests: #5585

[Jinming-Hu]

@ahsonkhan can we merge this PR as soon as possible? This is blocking our STG94 work.

[RickWinter]

@LarryOsterman can you please review as well.

[ahsonkhan]

can we merge this PR as soon as possible? This is blocking our STG94 work.

@Jinming-Hu We are waiting for this month's release cycle for May before merging this. So, I expect us to get this checked-in by eow. My current thinking is that we beta this as part of Core's release in June and GA it in July to align with when storage needs it.

[Jinming-Hu]

@ahsonkhan core release is done. Can we merge this now?