net: Zephyr's SNTP API time precision is not adequate

[pfalcon]

Currently available sntp_request() call returns time in seconds, completely ignoring fractional second value provided by SNTP. Second-level precision is clearly not adequate for the state of the art of technology.

Possible solutions:

• Returned precise time with fractional seconds as SNTP fixed-point number.
• Return it as POSIX struct timeval (converting fractional seconds to microseconds).
• Return it as POSIX struct timespec (converting fractional seconds to nanoseconds).

[pfalcon]

@d3zd3z, @galak: As we look into SNTP, we shouldn't pass by this issue.

[pfalcon]

Possible solutions:
Returned precise time with fractional seconds as SNTP fixed-point number.

#15600 chooses this solution, as the most obvious one, avoiding potentially expensive multiplications. Different users then can apply further calculations as they need.