Pluggable expression conversion for DF

[AdamGS]

This PR introduces an extendable interface for converting DataFusion physical expressions into Vortex ones, so that users can define their own behavior for UDFs or any other home-brewed physical expressions.

I think this is an OK way to expose the defaults for now, but would appreciate thoughts.

[codecov]

Codecov Report

❌ Patch coverage is 82.97872% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.99%. Comparing base (59bcf21) to head (ff59093).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-datafusion/src/convert/exprs.rs	88.46%	9 Missing ⚠️
vortex-datafusion/src/persistent/source.rs	22.22%	7 Missing ⚠️

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