[net-diag] introduce Server and Client classes (#8936)

This commit introduces `Server` and `Client` classes breaking the
`NetworkDiagnostic` module into two components. The `Server` responds
to queries and requests (handling Net Diag TMF commands), while
`Client` issues queries/requests and processes responses. The
`Server` is available under both FTD and MTD (which follows the
requirement of Thread spec). The client is enabled using newly added
`OPENTHREAD_CONFIG_TMF_NETDIAG_CLIENT_ENABLE` config option which is
also available as `OT_NETDIAG_CLIENT` CMake option or the autoconf
build switch `NETDIAG_CLIENT`. The client functionality is by default
enabled on Border Routers (tied to `CONFIG_BORDER_ROUTING_ENABLE`
config).

The previous `TMF_NETWORK_DIAG_MTD_ENABLE` config is now removed
along with its CMake `OT_MTD_NETDIAG` and autoconf switch. This
commit adds checks to trigger build error if the previous config
and/or its related CMake option are used.

script/check-size

@@ -142,7 +142,6 @@ size_nrf52840_version()
         "-DOT_LINK_RAW=ON"
         "-DOT_MAC_FILTER=ON"
         "-DOT_MESSAGE_USE_HEAP=ON"
-        "-DOT_MTD_NETDIAG=ON"
         "-DOT_NETDATA_PUBLISHER=ON"
         "-DOT_PING_SENDER=ON"
         "-DOT_SERVICE=ON"