feat: add script in the repo to setup envtest

[camilamacedo86]

This pr is just for we add the script that will be used in the solution to set up the env test locally for we are able to close #1599. See:

camilamacedo@Camilas-MacBook-Pro ~/go/src/sigs.k8s.io/kubebuilder (only-script) $ rm -rf testbin/
camilamacedo@Camilas-MacBook-Pro ~/go/src/sigs.k8s.io/kubebuilder (only-script) $ ./scripts/setup_envtest_bins.sh v1.18.2 v3.4.3
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 26.6M  100 26.6M    0     0  6759k      0  0:00:04  0:00:04 --:--:-- 6760k
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 47.8M  100 47.8M    0     0  7932k      0  0:00:06  0:00:06 --:--:-- 8369k
./testbin/kube-apiserver: replacing existing signature
./testbin/kube-apiserver: signed Mach-O thin (x86_64) [kube-apiserver-b9be6305b853bc12b0bf0ab6cf1b219fc4927cad]

[k8s-ci-robot]

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[camilamacedo86]

/assign @pwittrock

[pwittrock]

should this use the K8S_VER env var?

[camilamacedo86]

good catcher.

[estroz]

This value should be a script parameter ideally.

[pwittrock]

As a follow up consider defaulting these if they are unset. Maybe keep as env var

[pwittrock]

/lgtm

[estroz]

This needs documentation @camilamacedo86.

[camilamacedo86]

@estroz I still think that we need the new target in the Makefile that will call this script as you suggested.
See that users will no longer able to perform the quick start steps when the bins are no longer being shipped with kb. #686
We need to disengage the scaffold from the bins in kb as we need to solve this scenario when the installation is made via master and not the bin. See #1599. So, the docs will be done in this follow-up. We need to have the link for the script merged to continue on it.

[estroz]

Users shouldn't have to download kubebuilder's source to get this file. Its download and use should be documented in the quickstart. See #1600 (review).

Edit: I may be misunderstanding the problem this script is trying to solve. I did not realize upstream server binaries were being packaged with the kubebuilder binary.