Find a way to attach an existing table as a partition

Description

In CI/CD data partitioning PoC we prototyped a partitioning strategy that depends on an existing table being attached to a routing table as a partition zero.

Our initial findings in #351652 (comment 835068841) demonstrated that this can be done in the scale of GitLab.com when range partitioning is being used. However, it seems that this strategy might not be compatible with list partitioning: !80186 (comment 849286362)

Proposal

We need to find a safe way of attaching an existing table as a partition zero to a newly created routing table.

/cc @NikolayS @cheryl.li