[Feature Request]: Add "Default" Connection functionality

Having to select the connection against which to run a query after opening a `.sql` file is quite repetitive, It would be great if I could set a default connection for my project. For example, I could have a file "mssql.json" in the ".vscode" folder where I can set my default connection that will be used automatically and without ask when I click or the "Run" button. Then there could be another "Run" button, called "Run with ad-hoc connection" that would work is it is today, asking me which connection to use before running the query.

Here's an idea:


Start with the requested .vscode/mssql.json configuration and the existing Run button behavior described in the issue. Define how a project default connection is selected and how the separate ad-hoc connection action should preserve the current prompt-based behavior; done means both actions follow those distinct connection flows.

Written by the indexing model from the issue text.