Support inputs for parallel elements

Proposal

Currently inputs: supports a number type, but doing so fails in a parallel: element. It seems like a natural use for a numeric input.

spec:
    inputs:
        parallel-test-jobs:
            type: number
            default: 2
---
test:
    image: alpine
    parallel: $[[inputs.parallel-test-jobs]]
    script:
        - echo "Job $CI_NODE_INDEX of $CI_NODE_TOTAL"