[office-js-preview] (Outlook) Move signature APIs from Mailbox to item compose subclasses (#43127)
[office-js-preview] (Outlook) Move signature APIs from Mailbox to item compose subclasses (#43127)


mcliment/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped