[FEATURE] Add aliasing support to flatten command.

[currantw]

Is your feature request related to a problem?

Add support for aliasing in flatten command, consistent with OpenSearch Spark.

flatten <field> [As aliasSequence]

What solution would you like?

Update Lexer, Parser, Flatten, Analyzer, FlattenOperator, and all tests to support aliasing.

What alternatives have you considered?

None.

Do you have any additional context?

• Add flatten command to OpenSearch SQL
• Corresponding issue in OpenSearch Spark
• OpenSearch Spark documentation