Introduce cross project as-if-foss pipeline

What does this MR do and why?

Introduce cross project as-if-foss pipeline: https://gitlab.com/gitlab-org/gitlab-foss/-/pipelines/1121599641

• prepare-as-if-foss-branch is the job to prepare the as-if-foss/* branch.
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/5831884504
• prepare-as-if-foss-env is the job to prepare the variables and decide what jobs we want to run in the cross project FOSS pipeline.
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/5831884506
  • For example, if we run unit tests in the merge request, we also run unit tests in the FOSS pipeline. We run respective jobs based on the merge request pipeline.

Notes

• We do not need a sandbox project because all the code can be reviewed from the merge request. Nevertheless, developers should always be mindful when triggering a pipeline from a fork.
• To gradually switch the as-if-foss jobs to a new as-if-foss pipeline, we keep the original as-if-foss jobs, and introduce a new ~"pipeline:run-as-if-foss-cross-project" to trigger the new as-if-foss pipeline. It is not supposed to cover all the FOSS jobs in the first few iterations. When we're confident that it's all covered, we'll change pipeline:run-as-if-foss to trigger this new pipeline and delete as-if-foss jobs and remove the intermediate ~"pipeline:run-as-if-foss-cross-project".
• Not sure if we might hit into #436668 (closed) ?

TODO before merge

• Create a new label ~"pipeline:run-as-if-foss-cross-project" for this new trigger
• Create an access token AS_IF_FOSS in https://gitlab.com/gitlab-org/gitlab-foss/-/settings/access_tokens
  • With role developer and permission write_repository
• Put the above token as AS_IF_FOSS_TOKEN in https://gitlab.com/gitlab-org/gitlab/-/settings/ci_cd
  • With masked
• Set up maintenance scheduled pipelines to update FOSS cache or different versions of Ruby in https://gitlab.com/gitlab-org/gitlab-foss/-/pipeline_schedules

Related to #412717 (closed)