Add support for start-up Trace Flags and Env Vars to `sqlcmd create` command

[stuartpa]

Add support for something like:

sqlcmd create mssql --env "MSSQL_AGENT_ENABLED=True" --env "NAME=var" -- T9398

This would ensure trace flag 9398 (and the two --env vars) are passed into ./sqlservr every time the container instance starts.

[stuartpa]

This is an example, of how environment variables (to enable agent), and trace flags, are passed in.

docker run -e "ACCEPT_EULA=Y" -e "SA_PASSWORD="    -p 31436:1433 --name sql2019-new -h sql2019-new
   -e "MSSQL_PID=Evaluation" --env "MSSQL_AGENT_ENABLED=True" `
   -d mcr.microsoft.com/mssql/server:2019-latest /opt/mssql/bin/sqlservr -T 3205