Set custom initial joint state in Bullet simulation

[stephane-caron]

This change is essentially the same as https://github.com/tasts-robots/vulp/pull/33/files but for initial joint positions and velocities rather than base orientation. Currently the initial joint state is hard-coded to:

    bullet_.resetJointState(robot_, joint_index, 0.0);

We can pipe a custom joint state instead by extending BulletInterface::Parameters as in the PR.

[stephane-caron]

As discussed with @mariogpascual in #105 we should add velocities to the custom state in this proposal so that the full state of a simulation can be saved and restored.

[stephane-caron]

On a related note, #148 allows us to reset the initial base velocity in Bullet.

The corresponding PR in Vulp was upkie/vulp#61 (a similar change can be applied to reset joint states).

[stephane-caron]

Some progress in this direction with the new RobotState class in Upkie 3.2.0. There are joint_configuration and joint_velocity attributes, but for now they are placeholders not forwarded to the simulation. The next step will be to forward them in spine configuration dictionaries, and handle them in Vulp.