Store pipeline variables as encrypted pipeline_artifacts with dual writes

This issue involves implementation to enable dual writes for pipeline_variables as encrypted pipeline_artifact type both in DB and Object storage.

• Add pipeline_variables artifact type to Ci::PipelineArtifact model
• Add encryption support in PipelineArtifactUploader.
• Create CreateVariablesArtifactService to write variables to object storage.
• Integrate into CreatePipelineService, for enabling dual writes.
• Feature flag introduction in this change: pipeline_variables_artifact_write
• Write specs