Feature Store Dataset builder: add _to_athena_query method

[michaelneely]

Issue #, if available:

Description of changes:
Refactor feature_store.dataset_builder.to_csv_file() to call a new internal method _to_athena_query.

This accomplishes the following:

• A reduction in the complexity of the to_csv_file() function
• The possibility for users to extract the Athena query without actually running it, which unlocks use cases like transforming the Athena query into SparkSQL for those who would like to query the feature store via EMR or EMR Serverless.

Testing done:
Feature Store unit tests passing ✅

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[michaelneely]

@pintaoz-aws - Could you take a look at this PR? Thanks!