net: introduce scalar nanosecond time representation

Introduces a well-defined intermediate concept of syntonized scalar
nanosecond resolution time with overflow protection above low-level
counters/cycles/ticks and below higher level time abstractions
(timescales, calenders, etc.).

The rationale of this type has been extensively documented and
contrasted to already existing time representations to ensure that it
fills a well defined gap without overlap.

This change prepares for later changes in this change set that will
unify the usage of time across the network subsystem (RX/TX timestamps,
timed TX, CSL, scheduled reception windows, (g)PTP integration, etc.).

The type is EXPERIMENTAL and named net_time_t while it is not used in a
larger clock subsystems, the details of which are still being discussed
(see #60400 for details).

See
https://github.com/zephyrproject-rtos/zephyr/issues/19030#issuecomment-1597226731


for its embedding in a larger clock subsystem architecture relevant to
the network stack, IEEE 802.15.4 and the POSIX roadmap.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>