Allow configuration of the default user for EC2 SSM sessions

## Problem

The toolkit is configured to always uses ec2-user for Linux sessions created with the 'Connect VSCode to EC2' feature.
However the Open Terminal feature will use the ssm configured default, which is ssm-user.

## Expected behavior

Allow the user to specify the user for the 'Connect VSCode to EC2' feature. I'd like to be able to continue using ssm-user for some hosts when connecting VSCode.

Start by tracing the “Connect VSCode to EC2” feature and compare its session-user handling with the SSM default used by “Open Terminal.” Done means users can configure the session user for VSCode connections while hosts without an override continue using the existing behavior.

Written by the indexing model from the issue text.