Support use of array input type with expand_vars

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Proposal

expand_vars only works with the String input type. If you try to use expand_vars on another input type, the CI configuration is considered invalid. For example, with this CI/CD component...

spec:
  inputs:
    test:
      type: array
      default: 
        - 'Here is $VAR_1'
        - 'And here is $VAR_2'
---

test-job:
  script: echo $[[ inputs.test | expand_vars ]]

...you get this error message: "error in 'expand_vars' function: invalid input type: expand_vars can only be used with string inputs."

In #387632 (closed), where expand_vars was conceived, there's talk of adapting expand_vars to work with other data types, like Arrays: #387632 (comment 1550595104)

This is a proposal to implement that, and allow expand_vars to work with the Array input type.

Workarounds

I haven't identified a workaround. I tried adapting one described in this issue thread, but it failed with the same error message:

spec:
  inputs:
    test:
      type: array
      default:
        - "$[[ $VAR_1 | expand_vars ]]"
        - "$[[ $VAR_2 | expand_vars ]]"

---
test-job:
  script: echo "$[[ inputs.test ]]"