Add imports to force vendoring required codegen tools

[meln5674]

Description of the change

Add imports to force vendoring of tools required by ./hack/update-codegen.sh.

For whatever reason, only a subset of k8s.io/code-generator/cmd/ are vendored. This fails when k8s.io/code-generator/update-codegen.sh attempts to run

go install k8s.io/code-generator/cmd/conversion-gen k8s.io/code-generator/cmd/deepcopy-g
en k8s.io/code-generator/cmd/defaulter-gen k8s.io/code-generator/cmd/validation-gen

which fails because k8s.io/code-generator/cmd/validation-gen is not vendored.

Benefits

Currently, running make generate on main fails with

andrew:sealed-secrets$ make generate
go mod vendor
./hack/update-codegen.sh
cannot find module providing package k8s.io/code-generator/cmd/validation-gen: import look
up disabled by -mod=vendor
        (Go version in go.mod is at least 1.14 and vendor directory exists.)
make: *** [Makefile:50: generate] Error 1

With this change, make generate now succeeds.

Possible drawbacks

Applicable issues

N/A

Additional information

N/A