Allow forcing an interactive 'bst shell'

CB Baileyrequested to merge
bloomberg/bst/buildstream:interactive-bst-shell-cmd into master

By default, 'bst shell' only creates an interactive environment for running a shell if you do not provide a command.

Make it possible to run an interactive command via 'bst shell' by providing an '--interactive' option.

For background, I have been looking at invoking a debugger (gdb --interface=mi) from VSCode running on a Mac via bst shell --scope=run run inside a Docker container to debug a Linux x86_64 program.

Merge request reports