[.NET Core Debugging] Support setting/overriding browser URL

[philliphoff]

Currently the .NET Core debugging process detects whether a container exposes port 80 and, if so, then uses that to builds a URL to which the browser will be launched. However, if any other port is exposed, no browser will be launched.

We should expose a property in the debug configuration which allows the user to set or override the launch URL. (It might also be useful to allow it to be disabled even when exposing a well-known port.)

[bwateratmsft]

Fixed in #1245