Fix file variable expansion in downstream pipelines

Part 3 of #390252 (closed)

What does this MR do and why?

1. Prevent file variable expansion content in downstream pipeline variables.
2. Implicitly forwards file variable dependencies as file type to downstream pipelines when they are interpolated in a variable. This will ensure that the downstream pipeline has the file variable in the file system.

MR dependencies:

1. !121215 (merged) - adds a keyword argument to ExpandVariables to choose between expanding file variables or not.
2. !124580 (merged) - refactors downstream variables in Ci::Bridge.

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	Before	After
Child pipeline
Multi-project pipeline		Note that the file variable is created in the downstream project path:

How to set up and validate locally

1. Create a group test-group.
2. Create 2 projects test-group/main-project and test-group/downstream-project
3. In test-group, create a File variable GROUP_FILE_VARIABLE.
4. In test-group/main-project, create a File variable PROJECT_FILE_VARIABLE.
5. In test-group/main-project, create the following CI YMLs for the parent pipeline and child pipeline.
6. In test-group/downstream-project, create the following CI yml
7. Run pipeline manually on test-group/main-project, specifying a File variabel PIPELINE_FILE_VARIABLE.
8. Observe the results of the echo jobs
9. Enable feature flag :ci_prevent_file_var_expansion_downstream_pipeline
10. Repeat steps to run pipeline manually
11. Observe the results of the echo jobs

main-project CI YMLs

# .gitlab-ci.yml
variables:
  GROUP_FILE_VARIABLE: "$GROUP_FILE_VARIABLE"
  PROJECT_FILE_VARIABLE: "$PROJECT_FILE_VARIABLE"
  PIPELINE_FILE_VARIABLE: "$PIPELINE_FILE_VARIABLE"

echo:
  script:
    - echo "GROUP_FILE_VARIABLE $GROUP_FILE_VARIABLE"
    - echo "PROJECT_FILE_VARIABLE $PROJECT_FILE_VARIABLE"
    - echo "PIPELINE_FILE_VARIABLE $PIPELINE_FILE_VARIABLE"

trigger_child:
  trigger:
    include:
      - local: child-pipeline.yml

trigger_downstream_project:
  trigger:
    project: test-group/downstream-project

# child-pipeline.yml
echo:
  script:
    - echo "GROUP_FILE_VARIABLE $GROUP_FILE_VARIABLE"
    - echo "PROJECT_FILE_VARIABLE $PROJECT_FILE_VARIABLE"
    - echo "PIPELINE_FILE_VARIABLE $PIPELINE_FILE_VARIABLE"

downstream-project CI yml

echo:
  script:
    - echo "GROUP_FILE_VARIABLE $GROUP_FILE_VARIABLE"
    - echo "PROJECT_FILE_VARIABLE $PROJECT_FILE_VARIABLE"
    - echo "PIPELINE_FILE_VARIABLE $PIPELINE_FILE_VARIABLE"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #390252 (closed)