[AWSINTS-3515] feat(go-forwarder): add SNS

[ndakkoune]

What does this PR do?

Adds SNS as both standalone and S3 wrapping event.

Motivation

Testing Guidelines

Additional Notes

• As CloudTrail and EventBridge, we inject the SNS json event directly into the message.
• I'll add the tests after rebasing the the testify refactor PR.

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
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• This PR passes the integration tests (ask a Datadog member to run the tests)
• This PR passes the unit tests
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