[Bridges] add VariablePrimalStart support to ParameterToEqualTo and RSOCtoPSD

[odow]

Part of #684

[blegat]

The first two indices are multiplied by a 2x2 matrix (rotation by 90°) which makes this tricky since the starting value for one depend on the one for the other one. For get, it's not too bad, you can just query both. For set, it's a bit more tricky because when you get set the first one, you need to cache it it the bridge and way for the other one. Once you have both, you can set it to the model. We could throw an error in final_touch if we get set only one of them I misread, I thought it was SOC-RSOC bridge