Add support for helm extraFlags when installing Rancher

[thehejik]

• Adds support for optional extraFlags array when installing Rancher
• It is backward compatible so when extraFlags are omitted it will work as before

Can be used like this:

extraFlags := []string{
	"--set", "extraEnv[2].name=CATTLE_SKIP_HOSTED_CLUSTER_CHART_INSTALLATION",
	"--set-string", "extraEnv[2].value=true",
}

err := rancher.DeployRancherManager("my.awesome.domain", "latest", "devel", "2.9", "", "", extraFlags)
Expect(err).To(Not(HaveOccurred()))

https://go.dev/play/p/J8yUKpV-k6P

[sbulage]

LGTM 👍