Add async indexes for partitioned PK on ci_pipeline_variables

What does this MR do and why?

In this MR we follow steps 2 & 3 of the partitioning docs in order to prepare the ci_pipeline_variables table for partitioning.

We decided to create the indexes asynchronously since the table is quite large and the migration could take a while

There is a follow up MR to create the synchronous indexes here: !137777 (merged)

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Related to #429000 (closed)