Add bridge for SOS1 and SOS2 constraints

[ValentinKaisermayer]

Since Mosek does not support SOS1 and SOS2 constraints it would be nice to have a bridge to automatically reformulate these into simpler constraints.
Linke in the Mosek cookbook

[odow]

Closing in favor of jump-dev/MathOptInterface.jl#845 since this is best handled by an MOI bridge.