Make "kubernetes" multi-hop completion capable in Tramp

Make "kubernetes" multi-hop completion capable in Tramp

* lisp/net/tramp-container.el (tramp-skeleton-completion-function):
New defmacro.
(tramp-container--completion-function): Use it.
(tramp-kubernetes--completion-function): Use METHOD as argument.
Use `tramp-skeleton-completion-function'.
(tramp-skeleton-kubernetes-vector): New defmacro.
(tramp-kubernetes--current-context)
(tramp-kubernetes--current-context-data): Use it.
(tramp-completion-multi-hop-methods): Add "kubernetes".