Implement the "add index" handle and related task meta.

[zimulala]

The interface to be implemented

func NewHandle(ctx context.Context, se sessionctx.Context) (Handle, error) 
func (h *Handle) SubmitGlobalTaskAndRun(taskMeta proto.GlobalTaskMeta) (taskID int64, done chan struct{}, err error) 

Encode and Decode proto.GlobalTaskMeta

Implement

Transform the DDL job(type of add index, ref: model.BackfillMeta) to GlobalTaskMeta.

Reference

framework/handle.go
proto/task: SimpleNumberGTaskMeta