Shell API and CLI: Add option for staging additional runtime targets

[nathanwilliams-ct]

This enables users to add additional functionality such as debug tooling in the shell sandbox,
without needing to modify the target element. This is achived through introducing a new option to
shell.

All existing API and UX is maintained, to not break existing scripts. An alternative design was
considered, to have the additional elements as positional arguments similar to the existing element,
but this would need manual parsing to handle the cases where -- is present and not
present, splitting based on a .bst suffix. This UX could be re-visited in future.

Example usage:
bst shell --with base.bst example.bst -- cat example.txt
Where:

• example.bst is a simple import element with no dependencies that imports a file called example.txt
• base.bst provides a basic alpine sysroot with a standard set of unix tooling (sh, df, cat etc).

Changes:

• Introduces test_with_other_targets integration test to the shell test suite.
• Adds --with cli option to the shell subcommand and updates it's documentation.
  • option can be used multiple times by caller, providing a list of targets.
• Extends the shell top level calling interface in Buildstream core to accept a list of
  other_targets
  • This is where the targets are loaded into elements and checked to make sure they are present
• Extends the shell element implementation to accept a list of other targets
  • This is where the other elements are staged and integrated into the sandbox

towards: #422