net: sntp: Add sntp_query() function with fractional precision

Existing sntp_request() function has a coarse integer seconds
precision,  discarding fractional part as returned by SNTP.
Deprecate it, and instead introduce sntp_query() function which
returns both integer and fractional seconds as a newly introduced
structure sntp_tstamp.

Fixes: #15596

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>