Add a Redis connector (source + sink) #799

What & why

The project already has a Redis store (a state backend) but no Redis
connector (treating Redis as a data source/sink). They serve different roles:
the store backs the engine's internal state, while a connector backs DSL table
reads/writes. Redis as a dimension table or result cache is very common.

项目已有 Redis store(状态后端),但没有 Redis connector(把 Redis 当数据源/汇)。
两者职责不同:store 服务于引擎内部状态,connector 服务于 DSL 建表读写。Redis 作维表/结果缓存很常见。

The task

Create geaflow-dsl-connector-redis: the sink supports writing string/hash, the
source supports scanning by key pattern.

新建 geaflow-dsl-connector-redis,sink 支持写 string/hash,source 支持按 key 模式扫描读取。

Where to look / 怎么做

1. References: the minimal geaflow-dsl-connector-console (sink only) and the
   connection-managing geaflow-dsl-connector-elasticsearch.
2. Reuse the Redis client already in the project — check which client
   geaflow-store-redis uses and stick with it to avoid pulling in a new dependency.
3. Implement connector / source / sink + SPI registration + pom + docs + tests.

Done when

• Both source and sink work, supporting string and hash (at least one, with explanation)
• Reuses the existing Redis client dependency, no duplicate dependency introduced
• Testcontainers integration test
• CN + EN docs + examples
• checkstyle / RAT pass