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

openair2/E2AP/RAN_FUNCTION/CUSTOMIZED/ran_func_slice.c

@@ -0,0 +1,47 @@
+#include "ran_func_slice.h"
+#include "../../flexric/test/rnd/fill_rnd_data_slice.h"
+#include <assert.h>
+#include <stdio.h>
+
+void read_slice_sm(void* data)
+{
+  assert(data != NULL);
+//  assert(data->type == SLICE_STATS_V0);
+
+  slice_ind_data_t* slice = (slice_ind_data_t*)data;
+  fill_slice_ind_data(slice);
+}
+
+void read_slice_setup_sm(void* data)
+{
+  assert(data != NULL);
+//  assert(data->type == SLICE_AGENT_IF_E2_SETUP_ANS_V0 );
+
+  assert(0 !=0 && "Not supported");
+}
+
+sm_ag_if_ans_t write_ctrl_slice_sm(void const* data)
+{
+  assert(data != NULL);
+//  assert(data->type == SLICE_CTRL_REQ_V0);
+
+  slice_ctrl_req_data_t const* slice_req_ctrl = (slice_ctrl_req_data_t const* )data; // &data->slice_req_ctrl;
+  slice_ctrl_msg_t const* msg = &slice_req_ctrl->msg;
+
+  if(msg->type == SLICE_CTRL_SM_V0_ADD){
+    printf("[E2 Agent]: SLICE CONTROL ADD rx\n");
+  } else if (msg->type == SLICE_CTRL_SM_V0_DEL){
+    printf("[E2 Agent]: SLICE CONTROL DEL rx\n");
+  } else if (msg->type == SLICE_CTRL_SM_V0_UE_SLICE_ASSOC){
+    printf("[E2 Agent]: SLICE CONTROL ASSOC rx\n");
+  } else {
+    assert(0!=0 && "Unknown msg_type!");
+  }
+
+  sm_ag_if_ans_t ans = {.type = CTRL_OUTCOME_SM_AG_IF_ANS_V0};
+  ans.ctrl_out.type = SLICE_AGENT_IF_CTRL_ANS_V0;
+  return ans;
+
+}
+
+