[avro] Add Either typeclass support

[RustedBones]

Support scala Either as a avro type union

[codecov[bot]]

Codecov Report

:x: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 95.31%. Comparing base (2c10ba9) to head (bb2dcee). :warning: Report is 46 commits behind head on main.

Files with missing lines	Patch %	Lines
avro/src/main/scala/magnolify/avro/AvroType.scala	88.88%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1004      +/-   ##
==========================================
- Coverage   95.34%   95.31%   -0.03%     
==========================================
  Files          56       56              
  Lines        1978     1987       +9     
  Branches      154      157       +3     
==========================================
+ Hits         1886     1894       +8     
- Misses         92       93       +1     

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