Move default detectors ownership to OSS (#4257)

This directory holds only detector information, but it's in `/engine`, so its codeownership has been incorrectly assigned to non-OSS teams. Moving it to a detector-related spot created an import cycle that I don't want to deal with right now, so instead I'm just tweaking CODEOWNERS.

Co-authored-by: Amaan Ullah <[email protected]>

Author: rosecodym

CODEOWNERS

@@ -20,6 +20,8 @@ pkg/detectors/ @trufflesecurity/OSS
 pkg/common/ @trufflesecurity/OSS
 pkg/custom_detectors/ @trufflesecurity/OSS
 pkg/analzyers/ @trufflesecurity/OSS
+pkg/engine/defaults/defaults.go @trufflesecurity/OSS
+pkg/engine/defaults/defaults_test.go @trufflesecurity/OSS
 
 # critical detectors
 pkg/detectors/aws/ @trufflesecurity/backend