Allow configuration of the default user for EC2 SSM sessions

[nbaws]

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.

[arminanton]

Relates to #7452
Might be solved by: #7455