Command optional exec style

[javierfreire]

Checklist

• make test-all (UNIX) passes. CI will also test this
• unit and/or integration tests are included (if applicable)
• documentation is changed or added (if applicable)

Description of change

This PR completes the work done by @berney on #871

This change allows specifying a command to be executed exec style without a shell.
This change is backwards compatible, and existing goss configuration files should continue to work as expected.
This change is needed for operating in environments where a shell doesn't exist, such as testing a distroless docker image.
The syntax is inspired by the Dockerfile RUN, ENTRYPOINT, CMD instructions that can take either a string (shell style), or a list (exec style). This allows specifying arguments that contain spaces, etc, as the argument list is explicit.

Shell syntax (unchanged from before):

command:
  figlet:
    exit-status: 0
    exec: figlet test

This would be wrapped and executed as sh -c "figlet test"

Exec syntax (this change introduces):

command:
  figlet:
    exit-status: 0
    exec:
      - figlet
      - test

This would be executed directly, without a shell, as figlet test, where figlet is arg 0 and test is arg 1.

The above could use alternative yaml syntax:

command:
  figlet:
    exit-status: 0
    exec: [figlet, test]

---

📚 Documentation preview 📚: https://goss--1016.org.readthedocs.build/en/1016/

[javierfreire]

@aelsabbahy I completed #871 with the changes required. The integration tests are failing, but I think that's due to the code in the master branch, right? I have the same errors as in this #1013 (actions).

[javierfreire]

@aelsabbahy When you have a moment, I'd like to hear your thoughts on this.