Changing design for compilation signature

### The feature, motivation and pitch

I'm working on a robotics project that requires verifying whether two separate systems are using the same MuJoCo model. The signature field in the mjModel structure, which represents the compilation signature, seems ideal for this purpose.

However, I've noticed that even when the models differ in object positions, masses, or inertias, the compilation signature remains unchanged. It would be very helpful if this signature could reflect such differences, resulting in a more uniquely identifying compilation signature.

### Alternatives

Rewrite the mjCModel::Signature function to be dependent on the changed parameters.

### Additional context

_No response_