Follow-up from "Allow to use cross project pipelines"

The following discussion from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2017/diffs#note_35341320 that should be addressed:

In this case, we don't have trigger_request.variables so we have nowhere to store them.

We seem to have to create: ci_pipeline_variables (attached to the pipeline) that would hold all dynamic variables that are created by external sources:

ci_pipeline_variables
- pipeline_id
- key
- value

We should then migrate trigger_request.variables to use ci_pipeline_variables.

Please make sure that this file is backported to CE, the CE code is as much as possible and similar to EE version.

Here the difference would be in CE missing support for job_token triggering.

Migration is a big word, we don't have to migrate. Let's just start using new architecture for trigger_request.

For migrating the data we would have to:

> Ci::TriggerRequest.where.not(variables: nil).count
=> 209829

Maybe background migration would be good here?