feat: AWS SNS Sink

[jshlbrd]

Description

• Adds a sink for AWS SNS
  • Adds new internal package for SNS (internal/aws/sns)
  • Adds a new deployment example for publish/subscribe patterns (examples/aws/pubsub)
• Refactors several sink methods (non-breaking change)
• Fixes recursive returns in the Kinesis Firehose and SQS internal packages
• Adds new TODOs for v1.0.0

Motivation and Context

Adding support for pub/sub messaging patterns via SNS seemed like a natural fit for the project, and I've already added documentation to the v0.9.1 beta that shows some example architectures.

While working on this I came across the other items described above -- not all directly related to SNS, but otherwise all are improvements in the sink package.

How Has This Been Tested?

The new pub/sub example was integration tested in a development AWS account.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.