core/timer: Introduce RandomOffsetSec= knob (#36437)

This is like RandomDelaySec, but it doesn't reset whenever the manager
restarts.

Fixes https://github.com/systemd/systemd/issues/21166

src/core/timer.h

@@ -38,7 +38,8 @@ typedef struct Timer {
         Unit meta;
 
         usec_t accuracy_usec;
-        usec_t random_usec;
+        usec_t random_delay_usec;
+        usec_t random_offset_usec;
 
         LIST_HEAD(TimerValue, values);
         usec_t next_elapse_realtime;