CI YAML: Allow extends to merge values if they're arrays

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Problem to solve

We want to have an easy way to merge arrays when using extends.

As per https://docs.gitlab.com/ee/ci/yaml/#extends, "GitLab will Not merge the values of the keys".

Intended users

• Sasha (Software Developer)
• Devon (DevOps Engineer)
• Simone (Software Engineer in Test)
• Allison (Application Ops)

Further details

Proposal

Input:

.tests:
  script: rake test
  stage: test
  only:
    refs:
    - /^staging/

rspec:
  extends:
  - section: .tests
    strategy: merge # default: overwrite
  script: rake rspec
  only:
    refs:
    - /^preprod/

Actual output, assuming extends: .tests:

rspec:
  script: rake rspec
  stage: test
  only:
    refs:
    - /^preprod/ # It overwrote /^staging/

Expected output with introduction of extends:strategy: merge:

rspec:
  script: rake rspec
  stage: test
  only:
    refs:
    - /^staging/
    - /^preprod/ # Merged instead of overwritten

Links / references

Labels: ci-build devopsverify devopsconfigure ~"devops::release" (not sure exactly which one, things were simpler when CI/CD label exited 😆)

CC @markglenfletcher @ayufan @grzesiek @jyavorska