a friendly tool for scale-out

[lumianph]

Currently, the scale-out process consists of quite a few steps (refer to https://openmldb.ai/docs/zh/main/maintain/scale.html), and is error-prone. We need a friendly tool to help to do the scale-out (maybe also scale-in), it should have the functions:

• one command line to finish the startup of a new tablet, and migrate a specific partition to the new tablet
• an auto-rebalancing strategy that automatically migrates suitable partitions to the new tablet