[SpringForceField] Add stiffness matrix computation to JointSpringForceField

[alxbilger]

Supersedes #5895.

Blind attempt to implement buildStiffnessMatrix in JointSpringForceField. The implementation is based on addDForce, and in particular the method addSpringDForce.

I tested the scene JointSpringForceField.scn in the examples, but I suspect a problem in the scene. There are no unit tests, so this implementation couldn't be tested.

[with-all-scenes]

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[fredroy]

[ci-build][with-all-scenes]