[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/check-size

@@ -145,6 +145,7 @@ size_nrf52840_version()
         "-DOT_SRP_SERVER=ON"
         "-DOT_TIME_SYNC=ON"
         "-DOT_UDP_FORWARD=ON"
+        "-DOT_UPTIME=ON"
     )
 
     local thread_version=$1

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()

script/make-pretty

@@ -122,6 +122,7 @@ readonly OT_CLANG_TIDY_BUILD_OPTS=(
     '-DOT_COVERAGE=ON'
     '-DOT_LOG_LEVEL_DYNAMIC=ON'
     '-DOT_COMPILE_WARNING_AS_ERROR=ON'
+    '-DOT_UPTIME=ON'
 )
 
 readonly OT_CLANG_TIDY_CHECKS="\

script/test

@@ -67,6 +67,7 @@ build_simulation()
         "-DOT_SERVICE=ON"
         "-DOT_SRP_CLIENT=ON"
         "-DOT_SRP_SERVER=ON"
+        "-DOT_UPTIME=ON"
         "-DOT_THREAD_VERSION=${version}"
     )
 
@@ -275,6 +276,7 @@ do_build_otbr_docker()
         "-DOT_SLAAC=ON"
         "-DOT_SRP_CLIENT=ON"
         "-DOT_TREL=OFF"
+        "-DOT_UPTIME=ON"
         "-DOTBR_DUA_ROUTING=ON"
         "-DCMAKE_CXX_FLAGS='-DOPENTHREAD_CONFIG_DNSSD_SERVER_BIND_UNSPECIFIED_NETIF=1'"
     )