Feature request: Support DefinitionSubstitutions with StepFunctions updates

Currently when I make changes to an ASL file which is referenced from an `AWS::Serverless::StateMachine` that uses DefinitionSubstitutions, a full infra deploy is necessary, even if I didn't make changes to any `${References}`.

Expected behaviour:
If any references to substitution values are changed or added, then a full infra deploy may be necessary. Otherwise it'd be great if it was done as a fast non-infra update.