Backend: Support needs using inputs

Summary

In attempting to use inputs with needs and rules`, there is no mention in our help docs for how to implement this or if it is supported.

Example

Define some input parameters using spec:inputs and then attempt to refer to those inputs using needs or rules in the CI/CD. Example:

spec:
  inputs:
    TEST_JOB_NAME:
      description: Name of the E2E test
      type: string
    TEST_JOB_DEPENDENCY:
      description: Other CI jobs to wait for before executing
      type: string
      default: ""
---

$[[ inputs.TEST_JOB_NAME ]]:
  stage: test
  needs:
    - $[[ inputs.TEST_JOB_DEPENDENCY ]]
  image: ${CI_EXECUTOR_IMAGE}
  script:
    - env

This results in the error: Unable to create pipeline - job:[project]:needs:need config can't be blank

Proposal

Currently, we unfortunately don't support being able to pass needs in through inputs for the test job and if no needs are passed in to default to not having needs. The reason is that there's no way to have the needs: keyword on a job without specifying a need or [].

The pipeline should be created without error, or an error message is displayed that provides information on what can be changed to allow this pipeline to execute.

Confirm purpose and User Reception (how does this benefit the user?)

Links/References