[Outlook] (LaunchEvent) Add MessageAttachments as a child element

[learn-build-service-prod[bot]]

Learn Build status updates of commit fccc869:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
docs/manifest/launchevent.md	:white_check_mark:Succeeded	View (>=common-js || >=outlook-js-1.1 || >=excel-js-1.1 || onenote-js-1.1 || >=powerpoint-js-1.1 || visio-js-1.1 || >=word-js-1.1)

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