Add go mod verify script and pre-submit

[jpbetz]

go.mod should not be allowed to fall out of sync with vendor/modules.txt.

A hack/verify-vendor.sh style script should be added and then a verify presubmit should be added that calls this verify script.

[jpbetz]

/help

[k8s-ci-robot]

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[jpbetz]

Alternatively, could we drop vendor from this repo?

cc @Jefftree

[Jefftree]

Created a PR to remove vendor. #272

It seems more convenient than having to remember to vendor every time + adding additional CI scripts.