Add Tramp method "apptainer"

Add Tramp method "apptainer"

* doc/misc/tramp.texi (Top, Configuration): Add "Optional methods".
(Optional methods): New section.
(Inline methods) <androidsu, toolbox, flatpak>: These are optional
methods.
(Inline methods) <apptainer>: Add.

* etc/NEWS: New Tramp method "apptainer".
Some Tramp methods are optional.
Fix typos.

* lisp/net/tramp-androidsu.el (tramp-enable-androidsu-method):
New defun.  Call it when `system-type' is `android'.

* lisp/net/tramp-container.el (tramp-apptainer-program): New defcustom.
(tramp-apptainer-method): New defconst.
(tramp-apptainer--completion-function)
(tramp-enable-toolbox-method, tramp-enable-flatpak-method)
(tramp-enable-apptainer-method): New defuns.

* lisp/net/tramp.el (tramp-enable-method): New defun.

* test/lisp/net/tramp-tests.el (tramp--test-container-p):
Add "apptainer".
(tramp--test-supports-processes-p): Extend function.