Activate direct asynchronous processes for Tramp container methods

Activate direct asynchronous processes for Tramp container methods

* doc/misc/tramp.texi (Remote processes): Mention also
tramp-container.el.

* lisp/emacs-lisp/ert-x.el (ert-remote-temporary-file-directory):
Add `tramp-direct-async' to "mock" method.

* lisp/net/tramp-container.el
(tramp-methods) <docker, podman, kubernetes>: Add `tramp-direct-async'.

* lisp/net/tramp.el (tramp-methods): Adapt docstring.
(tramp-handle-make-process): Handle `tramp-direct-async'.

* test/lisp/net/tramp-tests.el (tramp-methods) <mock>:
Add `tramp-direct-async'.