[Feature] Support Fluss as ActionStateStore backend

[beryllw]

Search before asking

• I searched in the issues and found nothing similar.

Description

Summary

Currently only Kafka is supported as the external action state store backend for durable execution. This proposal adds Apache Fluss as an alternative backend.

Motivation

The Kafka-based implementation stores action states in an append-only log and rebuilds an in-memory HashMap on recovery by sequentially consuming the entire topic.
Add Apache Fluss as an alternative backend with efficient recovery via bucket-level offset tracking.

Scope

• Add BackendType.FLUSS and FlussActionStateStore implementation
• Add Fluss connection and authentication config options
• Add integration tests against an embedded Fluss cluster
• Update user documentation (configuration and deployment guides)

Are you willing to submit a PR?

• I'm willing to submit a PR!