Add new features in Outlook JS API

Please consider updating the Outlook JavaScript API to include the following features:

1. appendAsync feature: Unlike appendOnSendAsync, users need the ability to see the content in their emails/meetings before sending. While prependAsync exists, an appendAsync function would be beneficial for adding additional text/html to the current email being composed.

2. getSignatureAsync feature: Currently, there is no straightforward method to read the signature block. Implementing a getSignatureAsync function would streamline the process.

Reference: Office.Body interface - Office Add-ins | Microsoft Learn