Add ability to provide a cluster function in kubecontext segment

This makes it possible to customize the displayed cluster name, e.g.
`gke_project-name-1337_europe-west1-c_production` -> `production`

internal/p10k.zsh

@@ -2126,6 +2126,9 @@ prompt_kubecontext() {
         fi
       done
     fi
+    if [[ -n $POWERLEVEL9K_KUBECONTEXT_CONTEXT_FUNCTION ]]; then
+      ctx=$($POWERLEVEL9K_KUBECONTEXT_CONTEXT_FUNCTION $ctx)
+    fi
     _p9k_cache_set "$ctx" "$suf"
   fi