Introduce array index operator for CI/CD inputs

What does this MR do and why?

Introduce array index operator for CI/CD inputs

This change set introduces a new index operator [] for arrays when interpolating inputs.

For example (use it to try this out, too!):

spec:
  inputs:
    supported_versions:
      type: array
      default:
        - '2.0'
        - '1.0'
        - '0.1'
---

job:
  script:
    # Outputs: 'Latest version is 2.0'
    - echo 'Latest version is $[[ inputs.supported_versions[0] ]]'

It also supports the index operator on nested array values.

Changes are behind the ci_inputs_array_index_operator feature flag.

References

Refs Support array index operator `[]` for CI/CD inp... (#587657)

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