[Enhancement]: velocity estimation in joint_states for xs_sdk_sim

[cmower]

Describe the solution you'd like

In xs_sdk_sim, only the joint_states.position values are included - this is inconsistent with the real robot interface in xs_sdk_obj.cpp.

Platform

x-series arms

Is your feature request related to a problem? Please describe.

In some of my use-cases, I use the velocity from the real robot - I need to add hacks to my code in order to handle the simulation vs real cases.

Suggested code

I forked the repo, implemented backward differencing to estimate the velocity, and setup a PR #56

Additional Info

No response