Support launching a subset of services from a compose file

[bwateratmsft]

Many customers only want to launch some of the services in a compose file at any given time, let's support that scenario.

[bwateratmsft]

Our plan is to add a task for compose up / down, as well as a magic property for the compose up command that would prompt for a service list.

[bwateratmsft]

Related doc PR: microsoft/vscode-docs#4152