Fix issue #912: Support data_schema in CREATE EXTERNAL STREAM (Kafka)

[KathrynLin]

Summary

This PR fixes the issue where data_schema was not recognized in CREATE EXTERNAL STREAM for Kafka.

Changes

• Added data_schema to the ExternalStreamSettings.
• Updated InterpreterCreateQuery to extract and apply data_schema.
• Fixed schema auto-derivation from Avro format.
• Modified ParserCreateQuery to properly handle data_schema.

Testing

• Successfully created an external stream with data_schema:

  CREATE EXTERNAL STREAM example
  SETTINGS type = 'kafka',
           brokers = 'localhost:9092',
           topic = 'my-topic',
           data_format = 'Avro',
           data_schema = 'my_avro_schema';

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