[uptime] new feature to track OT instance uptime (in msec) (#6968)

This commit adds a new module `Uptime` which tracks the number of
milliseconds since OpenThread stack initialization as an `uint64_t`
value. It also adds public OT APIs to get the current uptime value
(either as the number of milliseconds or in human-readable string
format like "2 days 12:45:12.762"). A CLI `uptime` command is also
added. This feature can be enabled using the newly added config
option `OPENTHREAD_CONFIG_UPTIME_ENABLE` (or the related CMake
`OT_UPTIME` option).

script/cmake-build

@@ -100,6 +100,7 @@ readonly OT_POSIX_SIM_COMMON_OPTIONS=(
     "-DOT_LOG_LEVEL_DYNAMIC=ON"
     "-DOT_COMPILE_WARNING_AS_ERROR=ON"
     "-DOT_RCP_RESTORATION_MAX_COUNT=2"
+    "-DOT_UPTIME=ON"
 )
 
 die()