Adding GuardDuty to S3 source parser

[loonydev]

What does this PR do?

Adding GuardDuty to possible source on S3 event.

Motivation

We are using S3 as our GuardDuty findings storage. Then, we trigger Lambda with a bucket notification.
Unfortunately, source was not parsed correctly and was left as source:s3

Testing Guidelines

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
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• This PR passes the unit tests
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[ge0Aja]

This wasn't available as a source before refactoring the S3 handler
LGTM

[loonydev]

@ge0Aja Could I get approval for the CI workflow?