Support -n (number of messages) in z_sub_thr.c

[evshary]

Describe the feature

Now in z_sub_thr.c, the number of messages is fixed

zenoh-c/examples/z_sub_thr.c

Line 19 in be97279

#define N 1000000

However, it can be adjusted in the Rust implementation.
https://github.com/eclipse-zenoh/zenoh/blob/924394cfc26a19d851a9ff711e6d280e79a58b8d/examples/examples/z_sub_thr.rs#L104

It would be good to make the parameter adjustable. This would be especially helpful if we want to test the throughput with a large payload size.