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.
(cherry picked from commit
17c23a46c3d834e7dc9f153e86a51242b3867b74)