Support using external cli downloaders

Description of the new feature / enhancement

At the moment, we have 2 downloaders in the network settings wininet & do. With the downloader left at default (do), i get very low speeds especially on github. There has been numerous github issues regarding the same issue. Many people in these threads recommended setting the downloader to wininet to improve speeds. While this helped me improving the speeds little bit, its still nowhere near download speeds that i get on browser or say using aria2c. I think it will be very useful if a option is provided in the network settings to use a external command line based downlaaoder. i can then use aria2 by default with multiple connections to max out my download speed. i can certainly say that these external downloaders are much superior than the existing tools in providing consistent speeds without slowing down.

Locate the network settings and the existing wininet and do downloader entry points first. Define how an external command-line downloader such as aria2c would be configured and invoked, then verify the setting can select it and that downloads complete through the new path.