feat: make data types fields private

[aleksandarskrbic]

What changes are proposed in this pull request?

Fixes #1310

How was this change tested?

[codecov]

Codecov Report

❌ Patch coverage is 73.77049% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.76%. Comparing base (3bcd274) to head (04f844e).

Files with missing lines	Patch %	Lines
ffi/src/schema.rs	0.00%	6 Missing ⚠️
kernel/src/engine/ensure_data_types.rs	63.63%	2 Missing and 2 partials ⚠️
kernel/src/actions/mod.rs	0.00%	1 Missing ⚠️
kernel/src/engine/arrow_expression/apply_schema.rs	83.33%	0 Missing and 1 partial ⚠️
kernel/src/engine/arrow_expression/mod.rs	0.00%	0 Missing and 1 partial ⚠️
kernel/src/expressions/scalars.rs	50.00%	1 Missing ⚠️
kernel/src/scan/data_skipping.rs	80.00%	0 Missing and 1 partial ⚠️
kernel/src/transaction/mod.rs	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1491      +/-   ##
==========================================
- Coverage   84.76%   84.76%   -0.01%     
==========================================
  Files         122      122              
  Lines       33032    33037       +5     
  Branches    33032    33037       +5     
==========================================
+ Hits        28000    28004       +4     
- Misses       3659     3660       +1     
  Partials     1373     1373              

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[nicklan]

lgtm, thanks!