Add support for custom public key file in kube updater

[hugoShaka]

Since we moved the dev releases out of the prod registry I cannot test the updater anymore agianst dev releases.
This PR adds a flag to pass a file that will contain the staging public key.

[marcoandredinis]

Do we have public key for the stage env?
If we do, we could hard code that key instead of allowing a custom one.

[marcoandredinis]

We could use os.ReadFile instead

Suggested change

	file, err := os.Open(publicKeyFile)
	if err != nil {
	ctrl.Log.Error(err, "Failed to open public key file.", "file", publicKeyFile)
	os.Exit(1)
	}
	defer file.Close()
	key, err := io.ReadAll(file)
	if err != nil {
	ctrl.Log.Error(err, "Failed to read public key file.", "file", publicKeyFile)
	os.Exit(1)
	}
	key, err := os.ReadFile(publicKeyFile)
	if err != nil {
	ctrl.Log.Error(err, "Failed to read public key file.", "file", publicKeyFile)
	os.Exit(1)
	}

[tigrato]

Do we have public key for the stage env? If we do, we could hard code that key instead of allowing a custom one.

Yeah, I agree with Marco. We should hardcore both keys even if stg is not enabled by default