Defaults to ScalaBigDecimalTaggedGen in the idl-plugin

[fedefernandez]

Fixes #468

[codecov]

Codecov Report

Merging #485 into master will decrease coverage by 10.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #485       +/-   ##
==========================================
- Coverage   75.32%   65.2%   -10.12%     
==========================================
  Files          61      61               
  Lines         851     983      +132     
  Branches       14      14               
==========================================
  Hits          641     641               
- Misses        210     342      +132

Impacted Files	Coverage Δ
...en/core/src/main/scala/avro/AvroSrcGenerator.scala	78.43% <ø> (ø)	⬆️
...idlgen/core/src/main/scala/SrcGenApplication.scala	0% <ø> (ø)	⬆️

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