Tramp's method "kubernetes" supports now optional namespace

Tramp's method "kubernetes" supports now optional namespace

* doc/misc/tramp.texi (Inline methods): Extend Kubernetes host
name syntax by an optional namespace.
(Kubernetes setup): Mention optional namespace part of the host name.

* etc/NEWS: Tramp's method "kubernetes" supports now optional namespace.
Fix typos.

* lisp/net/tramp-container.el (tramp-kubernetes-namespace):
Initialize with nil.  Adapt docstring.
(tramp-kubernetes--name-regexp): New defconst.
(tramp-kubernetes--host-name-regexp): Change in order to cover
also an optional namespace.  (Bug#59797)
(tramp-kubernetes--namespace): New defun.
(tramp-kubernetes--context-namespace): Use it.
(tramp-skeleton-kubernetes-vector): Add sanity check for
superfluous user name or port number.

(cherry picked from commit d2cb9f2bf6ef9f3fcd8c21455ca1f1e624e61bf4)