Cascade partition_id to Ci::Sources::Pipeline

Ref: #383073 (closed)

What does this MR do and why?

This MR allows multi-project pipelines to be persisted in different partitions as described in the blueprint.

It does 2 things:

• Introduce source_partition_id and persists the partition_id from source_job
• Propagates partition_id from Ci::Pipeline to Ci::Sources::Pipeline.

This needs to be wired up before redefining all FKs for ci_builds in order to partition this table.

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