Compliance pipeline configuration in not exposed in merged_yaml

Summary

The merged_yaml field of https://docs.gitlab.com/ee/api/lint.html#validate-a-projects-ci-configuration doesn't include the compliance pipelines configuration.

Steps to reproduce

Create a project with a .gitlab-ci.yml file
Create a Compliance pipeline project with a .compliance-gitlab-ci.yml file containing Security jobs for example
Apply the compliance pipeline to the project created in 1.
Go to "https://gitlab.example.com/api/v4/projects/:id/ci/lint" where id is the project id
The compliance pipeline configuration is not merged, and therefore we don't see the final pipeline configuration for this project

This API resource is used to fetch the merged yaml configuration, I don't think there's another resource we can use instead, but please correct me if I'm wrong here.

Example Project

TODO (<- @thiagocsf do you have a project using compliance pipeline to share here?)

What is the current bug behavior?

What is the expected correct behavior?

merged_yaml is a view of the final pipeline configuration

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Expand for output related to GitLab environment info


(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

Merge the compliance pipeline config (if exists) into merged_yaml before returning the json respond.