net: hostname: Add a function to set the hostname postfix as is.

Fixes: #72363

The existing function to set the postfix is converting postfix
string to hexadecimal. Adding a new function to handle a use case
where the provided postfix should be used as is without any conversion.

Signed-off-by: Vineeta S Narkhede <VineetaSNarkhede@Eaton.com>