fix(aws-events): ergonomics improvements to CloudWatch Events

[rix0rrr]

Fixes the following things for CloudWatch events:

• Support newlines in CloudWatch Events textTemplate as intended, by making a newline-separated list of JSON strings. Fixes CloudWatch events: multi-line target input template is broken #1514.
• jsonTemplate now accepts arbitrary objects. They will be JSONified automatically. Fixes Allow fixed string input to CW Event #1198.
• Explicitly implement IEventRuleTarget on stepfunctions StateMachine so that Java/.NET users can trigger StateMachines using CloudWatch Events. Fixes part of step-functions: implement IEventRuleTarget, add grant() methods #1275.

---

Pull Request Checklist

• Testing
  • Unit test added
  • CLI change?: manually run integration tests and paste output as a PR comment
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
• Title and Description
  • Change type: title prefixed with fix, feat 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.