PoC: Create SingleInterpolation classes

What does this MR do and why?

This MR creates a SingleInterpolation module that contains modified versions of the Interpolator, Template, and Config classes from the Interpolation module. The modifications implement a single interpolation process, where an entire YAML config is interpolated in one go.

• SingleInterpolation uses the Interpolation::Access, Interpolation::Block, and Interpolation::Context classes without any changes
• This MR adds a ci_single_interpolation feature flag that we can use for a percentage based rollout
• Since this is a PoC, it only adds specs to Yaml::Loader so we can test at an integration level that the interpolation is working as expected
• This MR includes a new ArrayInput so we can test that single interpolation works for arrays

Local Testing

1. Create a file included.yml in the root directory of a project

spec:
  inputs:
    after_script:
    allow_failure:
      type: boolean
    cleanup:
    deploy_job_name:
    deploy_script:
      type: array
    parallel:
      type: number
    test_script_name:
---
.scripts:
  rspec:
    - echo "Runs rspec"
  cleanup:
    - echo "$[[ inputs.cleanup ]]"

test_job:
  after_script: $[[ inputs.after_script ]]
  parallel: $[[ inputs.parallel ]]
  script: !reference [.scripts, "$[[ inputs.test_script_name ]]"]

$[[ inputs.deploy_job_name ]]:
  allow_failure: $[[ inputs.allow_failure ]]
  script: $[[ inputs.deploy_script ]]

2. In the CI Editor, add the following configuration

# .gitlab-ci.yml

include:
  - local: included.yml
    inputs:
      after_script: "!reference [.scripts, cleanup]"
      allow_failure: true
      cleanup: Cleanup things
      deploy_job_name: inputs_deploy
      deploy_script:
        - echo "Staging deploy from input"
        - echo "Prod deploy from input"
      parallel: 8
      test_script_name: rspec

3. The parsed configuration should be:

---
".scripts":
  rspec:
  - echo "Runs rspec"
  cleanup:
  - echo "Cleanup things"
test_job:
  after_script:
  - echo "Cleanup things"
  parallel: 8
  script:
  - echo "Runs rspec"
inputs_deploy:
  allow_failure: true
  script:
  - echo "Staging deploy from input"
  - echo "Prod deploy from input"

References

Benchmarking:

• SingleInterpolation::Template !136075 (comment 1640719602)
• Loader::Yaml !136075 (comment 1640901614)

Issue: #411438 (closed)

Previous spike: !132045 (closed)