Allow strings and nested arrays of strings for before/after script

What does this MR do and why?

This Merge Request fixes #20957 (closed)

Nested "!reference[xxxx, script]" work
Nested "!reference[xxxx, before_script]" don't work
Nested "!reference[xxxx, after_script]" don't work

Looking at "Commands" and "Scripts" history, I wasn't able to find the reason for having 2 different objects

Therefore:

• "Scripts" are replaced by "Command"
• Tests are updated to change error message or remove case which are now allowed

Screenshots or screen recordings

CI configuration

default:
  image: alpine

test1:
  before_script:
    - echo "test1 before_script"
  script:
    - echo "test1 script"
  after_script:
    - echo "test1 after_script"

test2:
  before_script:
    - !reference [test1, before_script]
    - echo "test2 before_script"
  script:
    - !reference [test1, script]
    - echo "test2 script"
  after_script:
    - !reference [test1, after_script]
    - echo "test2 after_script"

test3:
  before_script:
    - !reference [test2, before_script]
    - echo "test3 before_script"
  script:
    - !reference [test2, script]
    - echo "test3 script"
  after_script:
    - !reference [test2, after_script]
    - echo "test3 after_script"

Unpatched Gitlab:
https://gitlab.com/guillaume.chauvel/ci-before-after-script/-/blob/944af7a4379b24d1139c037fc03ce72a77c9c06f/.gitlab-ci.yml
This GitLab CI configuration is invalid: jobs:test3:before_script config should be an array containing strings and arrays of strings

Updated GDK:

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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