Migrate trigger_request.variables to pipeline_variables

Description

We implemented Ci::PipelineVariables which is used for persisting variables for a single pipeline. And we made Trigger API(v4) use Ci::PipelineVariables instead of trigger_request.variables. We should migrate the data of trigger_request.variables to Ci::PipelineVariables with background migration.

v3 and v1 API still using trigger_request.variables. So it's better to wait until we remove the v3 and v1 code.

Proposal

Links / references

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13102

/cc @ayufan