Add a default user agent to REST source calls

[JohnMcPMS]

Change

Provides a default user agent header in the http helper class that the REST source code uses. It can be overridden by providing the user agent header in the headers argument.

The default format is:

winget-cli WindowsPackageManager/1.2.3 DesktopAppInstaller/10.11.12

where 1.2.3 is the client version as would be output by winget -v (without the v at the beginning) and 10.11.12 is the package version of the MSIX.

Also updates the WinINET downloader to use the same user agent string, but it is not overridable.

Validation

Added a unit test to ensure that the default was being set properly for both GET and POST.

Microsoft Reviewers: Open in CodeFlow

[jedieaston]

The WinINet downloader uses the user-agent winget-cli (see here), should that be switched to WPM/<version> too?

[JohnMcPMS]

The WinINet downloader uses the user-agent winget-cli (see here), should that be switched to WPM/<version> too?

I will take @denelon 's 👍 as an indication that he thinks so. I was indifferent, but will update the PR accordingly.

[JohnMcPMS]

Based on some internal discussion with @denelon, I've updated the actual user agent string (see PR description for new format). It will still be used in the same locations.