Make it easier to extract [common?] fields from events

[eladb]

The input transformation supported by eventRule.addTarget(target, inputTransformation) requires deep knowledge of how source events look.

For example, to trigger an SNS topic every time an EC2 instance changes its state, users currently need to write:

const instance = new ec2.Instance(this, 'MyInstance');
instance.onStateChange().addTarget(topic, {
  textTemplate: 'Instance <instance> changed state to <state>',
  pathsMap: {
    state: '$.detail.state', 
    instance : '$.detail.instance-id'
  }
});

This means that users must know exactly how an "EC2 Instance State-change Notification" looks like. Ideally that should be an implementation detail of the ec2.Instance class.

See #27 for context

[rix0rrr]

Wouldn't it be great though if we could do something like:

instance.onStateChange().addTarget(topic, {
  textTemplate: 'Instance <instance> changed state to <state>',
  pathsMap: {
    state: StateChangeEvent.detail.state,
    instance : StateChangeEvent.detail.instanceId
  }
});

Where I both

• a) discover what fields are available; and
• b) can't make mistakes.

Obviously, now I need to know I need to instantiate a StateChangeEvent, but even that could be fixed like so:

instance.onStateChange().addTarget(topic, {
  textTemplate: 'Instance <instance> changed state to <state>',
}).substituting(event => ({
    state: event.detail.state,
    instance: event.detail.instanceId
}));

[rix0rrr]

Although the latter will be hampered by the fact that jsii doesn't support Lambdas (...or does it?)