Feature Request: export vtctldclient VDiffShow summarization code

[mdlayher]

Feature Description

When executing vtctldclient vdiff show, a summary is generated based on the raw RPC outputs containing query rows, fields, etc. This summary is useful for humans using the CLI, but is also useful for automating the VDiff flow. Being able to import the summary logic as a library means that callers don't need to parse the entire VDiffShowResponse type to generate a report similar to vtctldclient.

Use Case(s)

Creating workflow automation tooling in Go, and being able to programmatically check the validity of workflows via VDiff.