feat: init gtp SM

Functionalities:

We introduce here the GTP service model, as know as NG, a custom made service model that extracts the following information (user id, gnb tunnel id, QoS flow indicator and upf tunnel id) from NGAP protocol.

Development includes:

• gtp service model folder
• xApp printing statistics
• xApp reporting message persistence in sqlite database
• Test frameworks
  • Three node test in C
  • All in one test
  • SM unit tests

Validation:

1. Compile code in build folder following README file
2. GTP SM test unit test:

• Go to flexric/build/test/sm/gtp_sm and run ./test_gtp_sm

3. All-in-one test:

• Go to flexric/build/test/agent-ric-xapp and run ./test_ag_ric_xapp

4. Three node tests

• Simulate agent
• Simulate near-RT-RIC
• Run ./flexric/build/examples/xApp/c/monitor/xapp_gtp_moni for c binding or ./examples/xApp/python3/xapp_gtp_moni.py