make `(read|write)_model_row` support custom schemas

[kleinschmidt]

Fixes #25 by allowing read/write model row to handle custom schemas:

• write_model_row takes an optional third positional arg which is the schema version to write (a la Legolas.write)
• read_model_row extracts the schema version and associated record type from the table metadata, like Legolas.read

I've added tests and updated the digits example, confirming that it can run end-to-end and produces the same results as the original. I re-committed the saved arrow file in order to be able to use the embedded schema info via read_model_row

[codecov[bot]]

Codecov Report

Merging #26 (e2e7b5c) into main (53c6778) will increase coverage by 0.24%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   93.87%   94.11%   +0.24%     
==========================================
  Files           2        2              
  Lines          49       51       +2     
==========================================
+ Hits           46       48       +2     
  Misses          3        3              

Impacted Files	Coverage Δ
src/LegolasFlux.jl	91.66% <100.00%> (+0.49%)	:arrow_up:

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