detail improvement odh notebook controller proxy context

[shalberd]

/kind feature

Why you need this feature:
code style improvement compared to #326

and

issue #291

Thank you @jiridanek

@harshad16 fyi

Describe the solution you'd like:
use context from NotebookWebHook and pass it in as context instead of TODO

kubeflow/components/odh-notebook-controller/controllers/notebook_webhook.go

Line 251 in 7401baf

func (w *NotebookWebhook) Handle(ctx context.Context, req admission.Request) admission.Response {

kubeflow/components/odh-notebook-controller/controllers/notebook_webhook.go

Line 229 in 7401baf

err := w.Client.List(context.TODO(), proxyResourceList)

proxyEnvVars does not need to be a global variable, especially since every call to ClusterWideProxyIsEnabled creates its content anew. Can we return the proxy env vars from the function?

https://github.com/opendatahub-io/kubeflow/pull/326/files/7401baff76e07f87b75a9ae4bdbb69431144722f#diff-1df80e242be90c94fce3ffb051a0bfe706fe924dab62438c7aa0a186b8067153R227

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

[jiridanek]

when we have go 1.21 available (release notes https://go.dev/blog/go1.21), there's opportunities to use https://pkg.go.dev/slices package

see the blog about it at https://go.dev/blog/generic-slice-functions