Add CallbackNodeStatus attribute

[odow]

See jump-dev/JuMP.jl#2127 (comment)

Add something like this, with docs and tests.

struct CallbackNodeStatus{CallbackDataType} <: MOI.AbstractOptimizerAttribute
    callback_data::CallbackDataType
end
is_set_by_optimize(:: CallbackNodeStatus) = true

@enum(
    CallbackNodeStatusCode,
    CB_NODE_STATUS_INTEGER,
    CB_NODE_STATUS_FRACTIONAL,
    CB_NODE_STATUS_UNKNOWN,
)

x-ref: jump-dev/JuMP.jl#2123