fix: correct the received timestamp

[YuanYuYuan]

Closes #337. This timestamp should be the one when received rather than the one when sent.

➜ ros2 topic echo /clock
clock:
  sec: 1734081513
  nanosec: 389786241
---
clock:
  sec: 1734081513
  nanosec: 414703983
---
clock:
  sec: 1734081513
  nanosec: 439739606
---

[clalancette]

I'm going to approve this one, and merge it, but I'll also have a follow-up PR since we need this in several other places as well.

[clalancette]

I'm going to approve this one, and merge it, but I'll also have a follow-up PR since we need this in several other places as well.

Oh no, never mind. That is only on the zenoh-cpp branch that we need this. This should be enough now; thanks!