feat: add dataTaskCompletionHandler to EventSource config

[jeongshin]

Hi Thanks for great library. It works well with my project!

Requirements

• [x] I have added test coverage for new or changed functionality
• [x] I have followed the repository's pull request submission guidelines
• [x] I have validated my changes against all supported platform versions

Related issues

I needed to read response data (json error message from server) when request error occurred while creating connection. But only responseCode is available for UnsuccessfulResponseError. So I added config to use completionHandler on creating urlSession dataTask.

Describe the solution you've provided

Added dataTaskCompletionHandler option.

Describe alternatives you've considered

I don't have any other idea to achieve this.

Additional context