Support overwrite mode for openmldb spark connector

**Describe the feature you'd like**

Now we can truncat the table data so that we may support overwrite mode for openmldb spark connector.

Look for the OpenMLDB Spark connector code, likely in a directory like 'spark-connector' or 'java'. Find where table truncation is implemented and understand the write modes. The goal is to add an 'overwrite' mode that truncates the table before writing. Check existing tests for the connector to see how write operations are validated.

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