Bridges: return copies of indices instead of the bridge's internal vector

[dourouc05]

jump-dev/MathOptInterface.jl#1344 (comment):

Some bridges return a vector for things like ListOfVariableIndices that they use internally. Since we have the (unstated???) contract that results returned from MOI.get are now the property of the caller, these should switch to returning copies. Anything else just seems liable to introduce bugs.