[feature-request] Add Webex message threads features

[sk4mi]

Hello,

This is possible to add a feature for webex receiver to allow to respond into a message thread when alert is resolved ?

Thanks

[simonpasquier]

I'm not an expert of the Webex notifier but I'd expect that Alertmanager would need the id of the initial notification. Right now, notifiers are fire-and-forget and don't store any state.

cc @gotjosh