Switch to using get_system_time_in_ns() function everywhere. (backport #354)

[mergify]

That way we have just a single place where this is defined, and everywhere can be sure they are getting the correct value in nanoseconds.

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This is an automatic backport of pull request #354 done by Mergify.

[mergify]

Cherry-pick of e7493d3 has failed:

On branch mergify/bp/jazzy/pr-354
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Changes to be committed:
	modified:   rmw_zenoh_cpp/src/detail/rmw_client_data.cpp
	modified:   rmw_zenoh_cpp/src/detail/rmw_service_data.cpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   rmw_zenoh_cpp/src/detail/rmw_subscription_data.cpp

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally