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>

CMakeLists.txt

@@ -383,6 +383,12 @@ endif()
 add_boolean_option(ENABLE_IMSCOPE OFF "Enable phy scope based on imgui" OFF)
 add_boolean_option(ENABLE_IMSCOPE_RECORD OFF "Enable recording IQ data for imscope" OFF)
 
+#########################
+##### E3 AGENT
+#########################
+set(E3_AGENT "OFF" CACHE STRING "O-RAN nGRG E3 Agent for dApps")
+set_property(CACHE E3_AGENT PROPERTY STRINGS "ON" "OFF")
+
 ##################################################
 # ASN.1 grammar C code generation & dependencies #
 ##################################################
@@ -1830,6 +1836,11 @@ if(E2_AGENT)
   target_compile_definitions(nr-softmodem PRIVATE ${E2AP_VERSION} ${KPM_VERSION} E2_AGENT)
 endif()
 
+if (E3_AGENT)
+    target_link_libraries(nr-softmodem PRIVATE e3ap)
+    target_compile_definitions(nr-softmodem PRIVATE E3_AGENT)
+endif()
+
 
 # force the generation of ASN.1 so that we don't need to wait during the build
 target_link_libraries(nr-softmodem PRIVATE