SLSCMN-16 SNS handler function types

[mwarman]

📢 Instructions

• Begin with a DRAFT pull request.
• Follow italicized instructions to add detail to assist the reviewers.
• After completing all checklist items, change the pull request to READY.

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🔧 Change Summary

List the changes included in this pull request.

• Issue: SLSCMN-16
• Create TS type for SNS handler function signature

📝 Checklist

Pull request authors must complete the following tasks before marking the PR as ready to review.

• Complete a self-review of changes
• Unit tests have been created or updated
• The code is free of [new] lint errors and warnings

🧪 Steps to Test

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1. No lint messages
2. Unit tests pass

🔗 Additional Information

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