Add option to ask for username

[Riezebos]

Is your feature request related to a problem? Please describe.
I want to put sqltools.connections in .vscode/settings.json to share with others. We use the same databases but have different usernames and passwords. I can remove password from the connection and it will be prompted, but if I remove the user it will leave the user empty.

Describe the solution you'd like
I'd like the same prompt that asks for the password to also ask for the username.

Describe alternatives you've considered
I can add docs telling people to copy a snippet to their own user settings json and add their username and password there, or I can tell them to use the gui. This leads to more errors, especially if SQL blocks are linked to a connection with a specific name.