Implemented by variable functions that can be evaluated asynchronously.
When a variable function class implements this interface, the Inedo.Extensibility.VariableFunctions.IAsyncVariableFunction.EvaluateAsync(Inedo.Extensibility.VariableFunctions.IVariableFunctionContext) method is always called instead of the Inedo.Extensibility.VariableFunctions.VariableFunction.Evaluate(Inedo.Extensibility.VariableFunctions.IVariableFunctionContext) method.