fix(webhook) #2614: added export for 'addAppMetadata'

[pushkarbanjare]

Summary

Fixes #2614

'addAppMetadata' already exists in '@slack/webhook', but it isn't exported from the package entry point, so 'import { addAppMetadata } from "@slack/webhook"' doesn't resolve. This adds the missing re-export, bringing it in line with
'@slack/web-api'.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

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[srtaalej]

hi @pushkarbanjare thank you for raising this issue and addressing it! I see that addAppMetadata is exposed in web-api but not in webhook. im looping in another team member to make sure addAppMetadata wasn't kept internal intentionally 🤔 will keep this thread updated with what i find !✨