Support async MoveTables vtctld operations

[nickvanw]

Summary

• Match the merged async MoveTables API so the Go SDK starts switch-traffic, reverse-traffic, and complete by returning an accepted operation id instead of waiting for long-running vtctld work to finish in request scope.
• Add generic vtctld operation polling on the existing Vtctld service so callers can wait for the final typed result without each command inventing its own polling client.
• Preserve backend validation semantics by mapping bad_request responses to ErrInvalid.

Changes

• change the three async MoveTables mutation methods to return VtctldOperationReference, including support for the merged rename_tables complete option
• add Vtctld.GetOperation plus request and response types for decoding the generic vtctld/operations/:id resource, keeping metadata and result as raw JSON for action-specific payloads
• update MoveTables and client tests to cover accepted responses, operation polling, and bad_request error handling

Notes

• This is a public SDK behavior change for those three MoveTables mutation methods; callers now need to poll Vtctld.GetOperation to retrieve the eventual result payload.