util: move utf8 utils to a separate header

This moves the declaration of the utf8 utils defined in lib/utils/utf8.c
in their own header. Main reason to do this is that the current setup
requried adding an include for sys/types.h in util.h, which can result
in a build falure due to a circular header depdenecy when using:

CONFIG_POSIX_API=y
CONFIG_NEWLIB_LIBC=y
_GNU_SOURCE

the loop and error are:

- include/sys/types.h:50: <- this is a NEWLIB one
- include/zephyr/posix/sys/select.h:9:
- include/zephyr/posix/posix_types.h:30:
- include/zephyr/kernel.h:17:
- include/zephyr/kernel_includes.h:25:
- include/zephyr/sys/atomic.h:18:

include/zephyr/sys/util.h:705:1:
error: unknown type name 'ssize_t'

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>