Allow adding InputPath and InputTransformer for notification targets

[Holi0317]

This is a Feature Proposal

Description

We have enabled sns notification for failed state machines and send the notification to slack via chatbot.

While the notification works, we want to customize the format that get sent to SNS. This can be done via customizing the InputPath and InputTransformer in the generated AWS::Events::Rule target.

Currently there is no way to include our change because the target generation code does not create those two properties at all.

Maybe all target type should be able to accept object as configuration and allow customizing the inputPath and inputTransformer?

Similar or dependent issues:

• 197 (not really dependent, but this issue introduced notifications)

Additional Data

• Serverless Framework Core Version you're using:
• The Plugin Version you're using:
• Operating System:
• Stack Trace:
• Provider Error messages: