Add iso_8601_realtime_microseconds time format for logging

Summary

• Adds new iso_8601_realtime_microseconds time_format option for log appenders
• Uses syscall(SYS_clock_gettime, CLOCK_REALTIME) to bypass libfaketime interception
• Enables accurate wall-clock timing of hived initialization during tests that use fake time

Usage

Configure appender with:

{"appender":"file","file":"hived.log","time_format":"iso_8601_realtime_microseconds"}