[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).

tests/fuzz/oss-fuzz-build

@@ -70,6 +70,7 @@
         -DOT_SRP_CLIENT=ON \
         -DOT_SRP_SERVER=ON \
         -DOT_THREAD_VERSION=1.2 \
+        -DOT_UPTIME=ON \
         ..
     ninja
 )