lib: cbprintf: add support for deferred formatting

In applications like logging the call site where arguments to
formatting are available may not be suitable for performing the
formatting, e.g. when the output operation can sleep.  Add API that
supports capturing data that may be transient into a buffer that can
be saved, and API that then produces the output later using the
packaged arguments.

[ Documentation and commit log from Peter Bigot. ]

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>