Rename set_parameter to set_attributes

[odow]

#2141 adds optimizer_with_attributes, so one can go:

model = Model(optimizer_with_attributes(Gurobi.Optimizer, "OutputFlag" => 0))

However, we still have set_parameter, which means

model = Model(Gurobi.Optimizer)
set_parameter(model, "OutputFlag", 0)

It would be more consistent if we renamed set_parameter to set_attribute, with the implication that passing a string was sugar for passing MOI.RawParameter.

In addition, we should add get_attribute.

[mlubin]

What about set_optimizer_attribute? We would also want to accept the same types of things as optimizer_with_attributes, i.e., both strings and MOI optimizer attribute objects.