Add SlackAPIConversationsHistoryOperator for conversations.history API

[yuseok89]

Slack conversation history is a valuable operational data source at a time when natural language and chat text are easy to fold into analytics, automation, and LLM workflows.
This PR adds SlackAPIConversationsHistoryOperator so DAG authors can fetch conversations.history in the same style as existing Slack API operators, with a fixed repeatable task pattern, templated parameters, and no extra Python glue.

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[potiuk]

Nice!