Direct Casadi solving for rate MPC

[joris997]

Instead of having to use acados, some users (like me :) ) may want to use the more intuitive casadi solver. This is a first PR for that, only containing the rate MPC.

I've decided to add a differentiator, euler and rk4 integration method to the spacecraft_rate_model. The MPC itself is located in spacecraft_casadi_rate_mpc.py. In the launch file (and therefore command line) it is now possible to call either acados or casadi for the rate mpc via the framework flag.
Current solvers: IPOPT and SQPMethod

Tested in SITL with

`ros2 launch px4_mpc mpc_spacecraft_launch.py mode:=rate namespace:=snap setpoint_from_rviz:=False framework:=casadi'

[Pedro-Roque]

Check these and change accordingly @joris997 , I'll approve it after :)

[joris997]

done @Pedro-Roque

[Pedro-Roque]

Great! If my test checks out, I'll merge.

[Jaeyoung-Lim]

Would it make sense that this would be a complete separate node, but sharing the models and formulations?

Or do you think this switch is general enough that we can switch back and forth?

I am a bit worried since the reason we went for acados was for simplicity. (You can see how little number of codes are needed to create a MPC node with this implementation.

[Pedro-Roque]

@Jaeyoung-Lim I think that the mpc_spacecraft should still be the entrypoint, and perhaps the MPC's need to be abstracted away. I'll start reviewing this PR and make some changes for clarity and ease-of-use.

@joris997 the time as finally come!

[Pedro-Roque]

@joris997 any updates on this?