Introduce trigger:forward for CI bridge jobs

What does this MR do and why?

By default, only YAML-defined bridge variables are passed to downstream pipelines. With the forward keyword, it is now available to pass manual pipeline variables to downstream pipelines.

• forward:yaml_variables is an existing behavior, by default it's true. When true, YAML-defined bridge variables are passed to downstream pipelines.
• forward:pipeline_variables is a new feature, by default it's false. When true, manual pipeline variables are passed to downstream pipelines.

This is behind a feature flag ci_trigger_forward_variables #355572 (closed).

Related to #213729 (closed)

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. Run the pipeline

4. Result

Parent Pipeline

Child 1

Child 2

Child 3

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