listPrivateChannels lists private channels that the installed bot can access.
It does not reveal private channels the bot has not joined or otherwise been
granted access to.
Example
automations/list-private-channels.automation.ts
Inputs
Output
Returns a signal withconversations: SlackConversation[] and
responseMetadata.nextCursor. Conversation records include the stable id,
name, visibility and membership flags, and available topic, purpose, creator,
creation, and member-count metadata.