add rocketchat event handler

please add rocketchat handler for events, making chat posts to user/channel

https://rocket.chat/docs/developer-guides/rest-api/chat/postmessage/

Start with the linked Rocket.Chat REST API documentation, focusing on the chat post message endpoint and its user/channel requirements. Then locate Kapacitor's existing event-handler entry points and determine how a Rocket.Chat handler would fit them. Done means events can create chat posts addressed to a user or channel, with behavior covered by the project's relevant tests.

Written by the indexing model from the issue text.