Add basic E2 agent through git submodule

- Add E2 agent that leverages FlexRIC e2ap library
- Add documentation in openair2/E2AP/README.md
- Currently, statistics exported are random

cmake_targets/build_oai

@@ -78,6 +78,8 @@ Options:
    Pass the supplied option verbatim to cmake.
 -d | --build-dir
    Sets build directory (will be <oai-root>/cmake_targets/<build-dir>/build)
+--build-e2
+   Enable the the E2 Agent
 -I | --install-external-packages
    Installs required packages such as LibXML, asn1.1 compiler, ...
    This option will require root password
@@ -197,6 +199,10 @@ function main() {
        -d | --build-dir)
             BUILD_DIR=$2
             shift 2;;
+       --build-e2 )
+             CMAKE_CMD="$CMAKE_CMD -DE2_AGENT=ON"
+             shift
+             ;;
        -I | --install-external-packages)
             INSTALL_EXTERNAL=1
             echo_info "Will install external packages"