[network-data] always support service registration at Leader (#4360)

A Leader should always be able to process service registration.

This commit:

- removes the OPENTHREAD_CONFIG_TMF_NETDATA_SERVICE_ENABLE feature
  flag around code for handling service registrations by the leader.

- support service ALOC forwarding by all router-capable devices.

- adds ability for any device to read service information from the
  network data.

src/cli/cli.cpp

@@ -42,6 +42,7 @@
 #include <openthread/icmp6.h>
 #include <openthread/link.h>
 #include <openthread/ncp.h>
+#include <openthread/netdata.h>
 #include <openthread/thread.h>
 #if OPENTHREAD_CONFIG_TIME_SYNC_ENABLE
 #include <openthread/network_time.h>
@@ -167,9 +168,7 @@ const struct Command Interpreter::sCommands[] = {
 #if OPENTHREAD_CONFIG_BORDER_ROUTER_ENABLE || OPENTHREAD_CONFIG_TMF_NETDATA_SERVICE_ENABLE
     {"netdataregister", &Interpreter::ProcessNetworkDataRegister},
 #endif
-#if OPENTHREAD_CONFIG_TMF_NETDATA_SERVICE_ENABLE
     {"netdatashow", &Interpreter::ProcessNetworkDataShow},
-#endif
 #if OPENTHREAD_FTD || OPENTHREAD_CONFIG_TMF_NETWORK_DIAG_MTD_ENABLE
     {"networkdiagnostic", &Interpreter::ProcessNetworkDiagnostic},
 #endif // OPENTHREAD_FTD || OPENTHREAD_CONFIG_TMF_NETWORK_DIAG_MTD_ENABLE
@@ -1723,7 +1722,6 @@ exit:
 }
 #endif
 
-#if OPENTHREAD_CONFIG_TMF_NETDATA_SERVICE_ENABLE
 void Interpreter::ProcessNetworkDataShow(int argc, char *argv[])
 {
     OT_UNUSED_VARIABLE(argc);
@@ -1742,6 +1740,7 @@ exit:
     AppendResult(error);
 }
 
+#if OPENTHREAD_CONFIG_TMF_NETDATA_SERVICE_ENABLE
 void Interpreter::ProcessService(int argc, char *argv[])
 {
     otError error = OT_ERROR_NONE;