Configure HttpClient initialization to default system proxy for sending HTTP requests

[dotnetian]

The code in the ServiceCollectionExtensions.cs has been updated to improve library's integration with system.

The previous behaviour for the library was ignoring the OS system-wide proxy settings, and always sending HTTP Requests directly. The code is updated so now if the OS supports this functionality and a system-wide proxy has been set, the library will also try to tunnel its HTTP requests through that.

[JKorf]

Hi, thanks for the PR; I didn't know about this functionality. Is the UseDefaultCredentials = true also necessary? The documentation specifically mentions that it doesn't affect proxy credentials.

[dotnetian]

I double-checked the official documentation, and you're right. UseDefaultCredentials can be removed as in this case it doesn't play any significant role. I see a merge conflict. Have you added this to the master branch yourself?

[JKorf]

Yes, after looking into this it seemed correct to add this. Since I was already making a new release I added the line myself.

Thanks for the PR though, let me know if you have any more improvements

[dotnetian]

I'm happy to help. But it would be amazing if you could merge this pull request, so I could become an "official" contributor to the project. Thank you for your hard work, and keep going!