Update CoverageReportDownloader to use System.Uri instead of string

[costin-zaharia-sonarsource]

CoverageReportDownloader and ICoverageReportDownloader are using strings instead of Uri.

String representations of URIs or URLs are prone to parsing and encoding errors which can lead to vulnerabilities. The System.Uri class is a safe alternative and should be preferred.

We need to refactor the downloader to use System.Uri.