Migrate VDiff rpc to VtctldServer

[ajm188]

Feature Description

We should have a gRPC endpoint to call VDiff on the new VtctldServer, similar to the vtctlclient VDiff vtctl command.

Use Case(s)

Programmatically calling Vdiff from the new clients.

Notes

This is going to involve a series of preperatory refactors to enable us to call TrafficSwitcher/VDiff code (vdiffer relies on traffic switcher) from outside of package wrangler and then it should be a simple final change to add the RPC endpoint.

[ajm188]

closing as dupe of #12155