Clean up E2E DataFusion + Optd demo

[connortsui20]

Problem

The demo was sort of messy and hard to read, so this PR refactors it.

Summary of changes

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 249 lines in your changes missing coverage. Please review.

Project coverage is 71.7%. Comparing base (285fe5f) to head (e321bd5).

Files with missing lines	Patch %	Lines
optd-datafusion/src/converter/into_optd.rs	0.0%	86 Missing ⚠️
optd-datafusion/src/mock.rs	0.0%	59 Missing ⚠️
optd-datafusion/src/optd_utils.rs	0.0%	46 Missing ⚠️
optd-datafusion/src/converter/from_optd.rs	0.0%	27 Missing ⚠️
optd-datafusion/src/lib.rs	0.0%	22 Missing ⚠️
optd-datafusion/src/converter/mod.rs	0.0%	9 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
optd-datafusion/src/converter/mod.rs	0.0% <0.0%> (ø)
optd-datafusion/src/lib.rs	0.0% <0.0%> (ø)
optd-datafusion/src/converter/from_optd.rs	0.0% <0.0%> (ø)
optd-datafusion/src/optd_utils.rs	0.0% <0.0%> (ø)
optd-datafusion/src/mock.rs	0.0% <0.0%> (ø)
optd-datafusion/src/converter/into_optd.rs	0.0% <0.0%> (ø)

[connortsui20]

Closing in favor of #38