Add pipeline schedule variables to forward:pipeline

What does this MR do and why?

In #213729 (closed), we implemented trigger:forward that allows users to have an option to decide sending some variables to downstream pipelines via bridge jobs. (!82676 (merged))

A user report (#357509 (closed)) reminded us that we don't pass schedule pipeline variables along with pipeline variables. In our "CI/CD variable precedence" list, they have the same priority. So we decided that we could consider pipeline schedule variables as pipeline variables.

This is still behind a feature flag ci_trigger_forward_variables #355572 (closed). We can disable the feature flag after merging this, then restart enabling.

Screenshots or screen recordings

1. Enable ci_trigger_forward_variables

Feature.enable(:ci_trigger_forward_variables)

2. Setup the pipeline

.gitlab-ci.yml

variables: # default variables for each job
  VARIABLE1: value 1

# default behavior
child1:
  trigger:
    include: .child-pipeline.yml

# forwarding pipeline variables
child2:
  trigger:
    include: .child-pipeline.yml
    forward:
      pipeline_variables: true

# not forwarding yaml variables
child3:
  trigger:
    include: .child-pipeline.yml
    forward:
      yaml_variables: false

.child-pipeline.yml

test:
  script:
    - echo $VARIABLE1
    - echo $VARIABLE2

3. Schedule a pipeline and run

4. Result

Parent Pipeline

Child 1

Child 2

Child 3

MR acceptance checklist

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