[feature request] Add nodepool flag when joining node with yurtadm

[luc99hen]

What would you like to be added:
Add a nodepool flag to yurtadm join command, yurtadm join --nodepool=xxxx

Why is this needed:
Currently, joining nodes and subsequently labeling them with the nodepool label is a tedious process. The nodepool label is crucial for numerous OpenYurt features pertaining to nodepool management. We can combine those two steps into one with this feature. Additionally, we can mandate the --nodepool flag for yurtadm join command.

others
/kind feature