Features: Support PortAttributes and ForwardPorts in devcontainer-feature.json

[warrenbuckley]

Context in the #devcontainer-community Slack

With me currently trying to reuse a global dotnet tool that is a fake/testing SMTP server, which is useful for testing code that sends out emails.

The dotnet tool is smtp4dev which runs a SMTP server listening on port 25 and a Web admin UI on port 5000 to view the emails received/sent out via the SMTP server.

With my feature I am building to use this dotnet tool, it would be great if the feature itself would set the ForwardPorts to 5000 and add a label and protocol into PortAttributes for visibility if you was to use the smtp4dev feature.