(Experimental) VReplication Workflows: cache workflow metadata in topo and use to filter target shards for vreplication commands

[rohit-nayak-ps]

Description

• Add workflow metadata to keyspace topo object
• Store metadata while creating a MoveTables object (currently only implemented for vtctlclient)
• Use the cached information, if available, to only contact primaries of participating shards for all workflow commands

Initial commit to check if any tests break due to this change.

Testing has been done by adding extra logging and checking that adding caching causes non-participating shards not to be contacted. Since we don't store stats for such calls, not sure how we can add tests for this without adding specific stats in vtctld.

Related Issue(s)

#14204

Checklist

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