Add constraint bridge for SOS1 and SOS2

[ValentinKaisermayer]

Since some solvers do not support SOS1 and SOS2 constraints it would be nice have an automatic reformulation.

Edit(odow): in MOSEK/Mosek.jl#182 @ohmsweetohm1 provided a helpful link to the Mosek cookbook.

[blegat]

In the cookbook, they say that a vector of binary variables in SOS1 can be bridged into sum(x) <= 1. However, this will not work for a vector of arbitrary variables. There is currently no way to specify that a bridge only works for binary variables.

[mlubin]

Agreed. SOS1 and SOS2 constraints are in general not represenable by mixed-integer linear programming. I don't see how we can make a bridge.

[odow]

Closing as won't fix.