feat: Create bigframes.bigquery.ml module

[google-labs-jules]

I have created the bigframes.bigquery.ml module and implemented the create_model function. I have also refactored the ModelCreationSqlGenerator to use the new centralized DDL generation logic. All tests are passing.

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PR created automatically by Jules for task 10823893811917836842 started by @tswast

[google-labs-jules]

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