Move new event targets into @aws-cdk/aws-events-targets

• CodePipeline
• EC2 task
• StepFunctions StateMachine

Invocation payloads are now under the control of the target classes, so
that targets for which the payload maps onto API calls (such as ECS,
CodeBuild, etc) can specify the API parameters directly as props.

EventTargetInput is a union class with three variants, allowing
serialization of strings, multiline strings and objects. Target inputs
support a special type of Token (EventField) which can be used to
refer to fields from the event, instead of value literals. A number of
predefined events and the fields that they have available have been
defined, so that accessing them becomes even more convenient
(StageChangeEvent, PhaseChangeEvent, ReferenceEvent).

To be able to use Tokens to implement EventInput substitution, add a
refactoring and more principaled separation of concerns in the Token
code. Resolution can now be more easily hooked, CloudFormation-aware
string concatenation is implemented using a plugin, and Token callbacks
receive an IResolveContext which they can use to resolve deeper (and
will reuse the same settings that the resolver was started with). We
should as good as be able to get rid of stack.node.resolve() in the
near future.

ALSO:

• Simplified LatestDeploymentResource to use existing logical ID
  overriding features.
• Add an onEvent() method to CloudTrail.
• Fix API misuse in the rendering of a CodePipeline, where Token
  rendering would lead to stateful side effects. Make Actions render
  out their region directly, if set, without requiring overrides.
• In ECS task assignPublicIp now defaults to undefined, instead
  of DISABLED, to align with service API.
• Fix a bug in Token resolution where Tokens returning fresh
  CfnReference objects upon resolution would fail to be detected
  during cross-stack reference analysis; CfnReference now has
  static methods that return singleton objects.
• Get rid of an extra "resolve" call in
  CfnResource.toCloudFormation(). We can now use PostProcessToken
  to apply the property renames and output validation we were originally
  doing the resolve for.

Fixes #2403, fixes #2404, fixes #2581.

BREAKING CHANGES

• @aws-cdk/aws-codepipeline.Pipeline is no longer an event target
  itself, use @aws-cdk/aws-events-targets.CodePipeline instead.
• @aws-cdk/aws-stepfunctions.StateMachine is no longer an event target
  itself, use @aws-cdk/aws-events-targets.SfnStateMachine instead.
• @aws-cdk/aws-ecs.Ec2RunTask has been renamed to
  @aws-cdk/aws-events-targets.EcsEc2Task.
• CloudFormationJSON.stringify() is now renamed to
  CloudFormationLang.toJSON().

Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
  • Design: For significant features, design document added to design folder
• Title and Description
  • Change type: title prefixed with fix, feat and module name in parens, which will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.