Move external PR pipeline creation to Sidekiq

What does this MR do?

Related to #336238 (closed)

This MR moves the external pull request pipeline creation to a new background job Ci::ExternalPullRequests::CreatePipelineWorker.

We expect that this will lower the average response time of POST /api/:version/projects/:id/mirror/pull.

This change is behind a feature flag ci_create_external_pr_pipeline_async (#338908 (closed))

For future reference

Last 7 days; https://log.gprd.gitlab.net/goto/20853e3b76ab39c205dbc6e526cbb696

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