VReplication: Add --shards flag to MoveTables/Reshard start and stop commands

[varundeepsaini]

Description

Adds support for the --shards flag to the start and stop commands for MoveTables and Reshard VReplication workflows. This allows users to start or stop workflows on a specific subset of shards rather than all shards at once.

Related Issue(s)

Fixes: #18949

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

No deployment notes. This is a CLI enhancement with no database migrations or breaking changes.

AI Disclosure

This PR was authored with assistance from Claude.

[vitess-bot]

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[promptless]

📝 Documentation updates detected!

New suggestion: Add --shards flag to MoveTables and Reshard start/stop commands

[promptless]

📝 Documentation updates detected!

New suggestion: Add changelog entry for VReplication --shards flag

[mattlord]

Thanks, @varundeepsaini !

There's more work to be done on the client side -- as we're not passing the shards in the request:

vitess/go/cmd/vtctldclient/command/vreplication/common/update.go

Lines 139 to 148 in 802b55e

	// The only thing we're updating is the state.
	req := &vtctldatapb.WorkflowUpdateRequest{
	Keyspace: workflowOptions.Keyspace,
	TabletRequest: &tabletmanagerdatapb.UpdateVReplicationWorkflowRequest{
	Workflow: workflowUpdateOptions.Workflow,
	Cells: textutil.SimulatedNullStringSlice,
	TabletTypes: textutil.SimulatedNullTabletTypeSlice,
	State: &state,
	},
	}

It's a different call path for workflow stop/start:

vitess/go/cmd/vtctldclient/command/vreplication/workflow/state.go

Lines 78 to 88 in 802b55e

	// The only thing we're updating is the state.
	req := &vtctldatapb.WorkflowUpdateRequest{
	Keyspace: baseOptions.Keyspace,
	TabletRequest: &tabletmanagerdatapb.UpdateVReplicationWorkflowRequest{
	Workflow: baseOptions.Workflow,
	Cells: textutil.SimulatedNullStringSlice,
	TabletTypes: textutil.SimulatedNullTabletTypeSlice,
	State: &state,
	Shards: shards,
	},
	}

We should also add some test coverage for this.