Setting starting values for vector variable bridges

[blegat]

This is working for FlipSignBridge since it just multiply by -1 but for a linear map that is not diagonal, we cannot set the starting values variable by variable.

https://github.com/jump-dev/MathOptInterface.jl/blob/5025b50de1d6cb29e6bbb0d26adfcf1b57b6a51c/src/Bridges/Variable/set_map.jl#L182-L192

We would need to cache the vector of starting values in the bridge and then do the linear map in final_touch.

Alternatively, this caching could be done by BridgeOptimizer to simplify the job of each bridge but that caching isn't needed for FlipSignBridge so it could be a waste.