common: extend timestamp interface for byte_buffer

* allow setting timestamp to a specific std::chrono::time_point
* add getter from raw tp

lib/include/srslte/common/common.h

@@ -155,6 +155,8 @@ public:
 #endif
   }
 
+  std::chrono::high_resolution_clock::time_point get_timestamp() { return tp; }
+
   void set_timestamp()
   {
 #ifdef ENABLE_TIMESTAMP
@@ -163,6 +165,12 @@ public:
 #endif
   }
 
+  void set_timestamp(std::chrono::high_resolution_clock::time_point tp_)
+  {
+    tp               = tp_;
+    timestamp_is_set = true;
+  }
+
   void append_bytes(uint8_t* buf, uint32_t size)
   {
     memcpy(&msg[N_bytes], buf, size);