etcd snapshot cropping and merge

[olderTaoist]

What would you like to be added?

• Etcd snapshot save operation should support with key prefix
• Etcd snapshot operation should support merge two snapshot file

Why is this needed?

for the k8s cluster whose resources are not split (-- etcd-serps-overrides), if want to split, when taking snapshots, should take snapshots according to the resource type (node/pod/lease/configmap), that is, the snapshots only contain part of the resource data. then create a new cluster (3379) using snapshot recovery. When there is a problem with the split cluster (3379), the data in the snapshot of the faulty cluster (3379) can be merged into the snapshot of the original resource cluster (2379).