build: add opt-in E3_AGENT build option for the O-RAN E3 agent

Introduce the E3_AGENT CMake option (default OFF) and the matching
--build-e3 flag in build_oai so the E3 agent for O-RAN dApps can be
compiled opt-in. Gate the E3AP subdirectory and the nr-softmodem
linkage behind the option, and register an E3AP log component
together with its legacy T-tracer message IDs.

The E3 interface (nGRG) lets the gNB exchange real-time reports and
controls with external dApps. Keeping it behind a build flag leaves
the default CI build unchanged and avoids pulling in the external
libe3 dependency unless the agent is explicitly requested.

Signed-off-by: Andrea Lacava <thecave003@gmail.com>

openair2/CMakeLists.txt

@@ -5,6 +5,10 @@ add_subdirectory(E1AP)
 if(E2_AGENT)
   add_subdirectory(E2AP)
 endif()
+if(E3_AGENT)
+  message(STATUS "Add E3 Agent capabilities")
+  #add_subdirectory(E3AP)
+endif()
 add_subdirectory(F1AP)
 add_subdirectory(LAYER2)
 add_subdirectory(M2AP)