feat(sns): add support for attribute key matching in message filtering

[jogold]

SNS message filtering now supports attribute key matching. This feature lets you create an Amazon
SNS subscription filter policy that matches incoming messages which contain an attribute key,
regardless of the attribute value associated with this key. This feature lets you offload additional
message filtering logic to Amazon SNS.

Update SubscriptionFilter to support this new feature.

See https://aws.amazon.com/about-aws/whats-new/2019/08/amazon-sns-message-filtering-adds-support-for-attribute-key-matching/

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[mergify]

Pull Request Checklist

• Testing

• Unit test added (prefer to add a new test rather than modify existing tests)
• CLI change? Re-run/add CLI integration tests

• Documentation

• Inline docs: make sure all public APIs are documented (copy & paste from official AWS docs)
• README: update module README
• Design: for significant features, follow the design process

• Title uses the format type(scope): text

• Type: fix, feat, refactor go into CHANGELOG, chore is hidden
• Scope: name of the module without the aws- or cdk- prefix or postfix (e.g. s3 instead of aws-s3-deployment)
• Style: use all lower-case, do not end with a period

• Description

• Rationale: describe rationale of change and approach taken
• Issues: Indicate issues fixed via: fixes #xxx or closes #xxx
• Breaking?: last paragraph: BREAKING CHANGE: <describe what changed + link for details>

• Sensitive Modules (requires 2 PR approvers)

• IAM document library (in @aws-cdk/aws-iam)
• EC2 security groups and ACLs (in @aws-cdk/aws-ec2)
• Grant APIs (if not based on official documentation with a reference)

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