Add a MongoDB connector

[Leomrlin]

What & why

GeaFlow already supports JDBC / Hive / ES / Neo4j, but there's no connector for
the most popular document database, MongoDB. It's a frequent community
request — Mongo can serve as both a vertex/edge data source and a result sink.

GeaFlow 已支持 JDBC/Hive/ES/Neo4j,但缺主流文档数据库 MongoDB。这是社区高频需求:
Mongo 既能做点/边数据源,也能做结果输出。

The task

Create a new module geaflow-dsl-connector-mongodb with a source (partitioned
reads, optional predicate pushdown) and a sink (batched writes).

新建模块 geaflow-dsl-connector-mongodb,实现 source(分区读、谓词下推可选)+ sink(批量写)。

Where to look / 怎么做

1. Copy the most complete references for scaffolding: streaming
   geaflow-dsl-connector-kafka, partition/format geaflow-dsl-connector-file,
   remote-store class geaflow-dsl-connector-elasticsearch.
2. Implement MongoTableConnector (TableReadableConnector +
   TableWritableConnector), MongoTableSource
   (init/open/listPartitions/getDeserializer/fetch), and MongoTableSink
   (init/open/write/flush/close).
3. SPI registration:
   src/main/resources/META-INF/services/org.apache.geaflow.dsl.connector.api.TableConnector.
4. Add the module to the parent pom.xml and to the connector aggregation pom.
5. Docs (CN + EN) + an examples GQL + an integration test.

Done when

• Both source and sink work, with basic connection config (uri / db / collection / batch size)
• listPartitions splits by shard/range to enable parallel reads
• Integration test included (Testcontainers can spin up MongoDB)
• CN + EN docs and one examples GQL
• Passes checkstyle and RAT (ASF header)

[KaustAbhinand]

Hello! I can work on this issue.

Could you mention how to run the tests and the expected results?

Thanks!

[johntomcat7408-cmyk]

Hi, I'd like to work on this issue.

@KaustAbhinand, are you still working on it?

For the first version, I plan to support bounded collection reads with explicit
range partitions, batched inserts, SPI registration, and integration tests.

Could you confirm whether this scope is acceptable and whether the documentation
should be updated here or in geaflow-website?

[KaustAbhinand]

Hi @johntomcat7408-cmyk, feel free to take it up :)