feat: implement threaded message strategy for Slack notifier

[santiagofn]

Part 4 of a series implementing threaded message support for the Slack notifier. Full picture: #5150. Depends on #5224, #5225, and #5226.

Summary

Implements the core threaded message strategy. When message_strategy: thread is configured, a parent summary message is posted first, and subsequent firing/resolved notifications are posted as thread replies. The strategy uses the notification log to persist the parent message's ts and channel across alert state changes, enabling correct threading even across alert manager restarts.

Also implements support for updating the parent message emoji and text on resolution when resolve_emoji or update_parent_on_resolve are configured.

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