feat: Add dsm support for sns->lambda

[michael-zhao459]

What does this PR do?

Add support for sns -> lambda in DSM. Actually, SNS->SQS-> lambda was also taken care of as no checkpoints need to
be set between the queues. So the SQS->lambda PR already solved that

Motivation

Lambdas were broken in DSM

Testing Guidelines

Wrote unit tests for added functionality

Additional Notes

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
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)