feat(flow): add Kalki-backed persistence checkpointer (#4644)

[yuweuii]

Summary

This PR implements issue #4644.

• Scope: [FEATURE] Kalki as a high-performance long-term memory checkpointer for agentic applications.
• Source branch: yuweuii:codex/issue-4644
• Commit: d589a40

Linked Issue

Closes #4644

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Note

Medium Risk
Adds a new non-default persistence backend that checkpoints and restores flow state (including async pending-feedback resume markers) via external gRPC service calls, which could affect flow recovery behavior and introduce dependency/compatibility issues with Kalki protos.

Overview
Adds an optional Kalki-backed flow persistence backend (KalkiFlowPersistence) that checkpoints flow state to a running Kalki service via gRPC and restores the latest checkpoint by querying and timestamp-sorting stored log payloads.

The new backend also supports async human-feedback resume by writing pending-feedback markers and clearing them via an append-only tombstone entry, and it is exported from crewai.flow.persistence and documented in the Flows persistence guide with a usage example.

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[greysonlalonde]

Hi! We are generally not accepting any external tool additions at the moment. Please see https://docs.crewai.com/en/guides/tools/publish-custom-tools