Add Tramp distrobox method

Add Tramp distrobox method

* doc/misc/tramp.texi (Inline methods) <distrobox>: Add.

* etc/NEWS: Mention Tramp distrobox method.

* lisp/net/tramp-container.el (tramp-distrobox-program): New defcustom.
(tramp-distrobox-method): New defconst.
(tramp-distrobox--completion-function)
(tramp-enable-distrobox-method): New defuns.  (Bug#71200)
(tramp-docker-method, tramp-dockercp-method)
(tramp-podman-method, tramp-podmancp-method)
(tramp-kubernetes-method, tramp-flatpak-method)
(tramp-apptainer-method, tramp-nspawn-method): Adapt docstring.

* test/lisp/net/tramp-tests.el (tramp--test-box-p): Rename from
`tramp--test-toolbox-p'.  Add distrobox.  Adapt callees.