Fix timestamp written by clock plugin

[xela-95]

This PR affects the clock plugin.

It fixes the format of the timestamp written on the /clock port to be what yarp requires.

[traversaro]

Thanks, for reference the related code in gazebo-yarp-plugins and YARP:

• https://github.com/robotology/gazebo-yarp-plugins/blob/bad19ba865278d38b7e93b31706e42ff487b0bb7/plugins/clock/src/Clock.cc#L167
• https://github.com/robotology/yarp/blob/2e513a64f2ffaa43b91c212c4bfbf20049b576b7/src/libYARP_os/src/yarp/os/NetworkClock.cpp#L103-L105

[xela-95]

xref to the issue with NetworkClock: https://github.com/robotology/yarp/issues/3096

[xela-95]

Merging! 🚀