[substrait] refactor consumer.rs

[vbarua]

Is your feature request related to a problem or challenge?

The changes in #13803 split the consumer code into many utility functions. To make it easier / clearer to add more functions in the future, we should split consumer.rs into multiple files. That would also be a good opportunity to add module level documentation.

See:

• feat(substrait): modular substrait consumer #13803 (comment)
• feat(substrait): modular substrait consumer #13803 (comment)

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