Return the JSON status object from the healthz endpoint

[mszostok]

Overview

(Discussed on the dev-team meeting on Dec 15)

In the #881 we introduce a dedicated Botkube heatlhz endpoint that returns 200 (OK).

As a part of this task, we should return JSON with information about Botkube core health:

• and enabled plugins (after Watch and respawn started plugins that exited #878)

• and communications platform statuses (if possible)

• Ensure the bot + plugin statuses can be used as a part of reportDeploymentHeartbeat GraphQL query

Reason

Provide visibility about Botkube and related components' health.