Make Parquet-SplittableDoFn opt-in, not opt-out

[clairemcginty]

No description provided.

[codecov]

Codecov Report

Merging #4700 (a70d43e) into main (1e58385) will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4700      +/-   ##
==========================================
+ Coverage   60.63%   60.93%   +0.29%     
==========================================
  Files         286      286              
  Lines       10457    10457              
  Branches      667      667              
==========================================
+ Hits         6341     6372      +31     
+ Misses       4116     4085      -31     

Impacted Files	Coverage Δ
...y/scio/parquet/read/ParquetReadConfiguration.scala	100.00% <100.00%> (ø)
...spotify/scio/parquet/read/ReadSupportFactory.scala	33.33% <0.00%> (-66.67%)	⬇️
.../com/spotify/scio/parquet/read/ParquetReadFn.scala	83.01% <0.00%> (-2.84%)	⬇️
...y/scio/values/PairSkewedSCollectionFunctions.scala	95.94% <0.00%> (+2.70%)	⬆️
...ify/scio/parquet/tensorflow/ParquetExampleIO.scala	88.88% <0.00%> (+7.40%)	⬆️
...la/com/spotify/scio/parquet/GcsConnectorUtil.scala	21.87% <0.00%> (+9.37%)	⬆️
.../com/spotify/scio/parquet/avro/ParquetAvroIO.scala	84.84% <0.00%> (+11.11%)	⬆️
...com/spotify/scio/parquet/types/ParquetTypeIO.scala	98.59% <0.00%> (+22.53%)	⬆️

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