Add `sqlcmd open code` support

[stuartpa]

Request from @yorek. Will flesh out details of what sqlcmd open code should do in this issue

[yorek]

Thanks @stuartpa. Ideally it should behave similarly to that sqlcmd open ads does. Open VS Code and connect to the database defined in the active context.

[shueybubbles]

This will be a good chance to refactor the tool code. We can define some command line templates indexed by tool name, with placeholders for connection parameters. Eventually we hope more tools will support reading the connection info by name from the sqlcmd config file, or support the new xml-based connection file format used by SSMS 19.1+