Missing API for dynamically changing DistanceJoint length

[TouSC]

In the Flame Forge2D joints documentation, it says:
The joint assumes that the maximum length doesn’t change during simulation. See DistanceJoint if you want to dynamically control length.

However, in the current flame_forge2d API I couldn’t find any way to change the length of a DistanceJoint during runtime.
In Box2D , b2DistanceJoint provides methods like SetLength(...), which allow dynamically controlling the joint’s rest length. But these do not seem to be exposed in the forge2d.

[spydon]

I think it should be possible to implement this without having to go through too much troubles, do you want to work on a PR for that @TouSC?