Allow writing output of plan files from terraspace all command

[samjsmart]

Summary

Add an --out=<path> option to the terraspace all plan command that will write the plan files from each stack to the specified directory, in the form of <path>/<stack>.plan or <path>/<stack>.json

Motivation

This will be useful within CI pipelines as it allows tooling, such as infracost, to be executed on every stack using the provided output files. Currently this can be achieved by running terraspace plan <stack> --out=<plan path>, however, this method does not leverage Terraspace's dependency resolution or stack discovery, and so each command must be manually added to CI configuration in the correct order. Having the ability to output all plan files will make integration with other Terraform tooling much easier.

Guide-level explanation

Usage:
  terraspace all plan

Options:
o, [--out=OUT]                              # Write the output(s) to path
y, [--yes], [--no-yes]                     # auto approve all batch commands
      [--exit-on-fail], [--no-exit-on-fail]  # whether or not to exit when one of the batch commands fails