Use custom agent header in axios web requests.

We should add a request header for user-agent that makes our web request worker singleton send the user agent header as 'vscode-dotnet-runtime' so we can see in telemetry where requests are coming from. We should add a small unit test to validate this and compare to see what the prior user agent header was in case there was protection already in place against the existing agent header for sending too many requests.

thank you @drewnoakes for the suggestion!