Support reader for openmldb-spark-connector

Now we can use Spark to write openmldb online storage but can not read data from online storage.

Look for the openmldb-spark-connector module in the repository, likely under connectors/. Examine the existing writer implementation to understand the interface with OpenMLDB online storage. The task is to implement a corresponding reader, which will require understanding Spark DataSource API and OpenMLDB's storage format. Check for any existing tests for the connector to see the expected behavior.

索引モデルが issue の本文から書いたものです。