Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

add a func to merge multiple Strategic Merge Patches into one patch

This PR is adding a function similar to https://github.com/evanphx/json-patch/blob/9fa11df836c7537e8ad3d1639fa0d396559a7bab/merge.go#L95-L100
It merges multiple SMPs and yield another SMP.

This PR has no risk to existing SMP code.

```release-note
NONE
```
/cc @apelisse
/assign @pwittrock