Add partition_id to Ci::PipelineArtifact

Ref: #430722 (closed)

What does this MR do and why?

This MR cascades partition_id from pipeline to Ci::PipelineArtifact.

We need to add this new column in order to redefine the FK association between ci_pipeline_artifacts and ci_pipelines.

TABLE "ci_pipeline_artifacts" CONSTRAINT "fk_rails_a9e811a466" FOREIGN KEY (pipeline_id) REFERENCES ci_pipelines(id) ON DELETE CASCADE

Why are we doing this?

In order to partition ci_pipelines all referenced FK must include the partition key as explained in our documentation.