Require import for kryo implicit fallback coder

[RustedBones]

• Only define coders in dedicated trait and avoid forward coder usage (trait mixed in Coder calling Coder.xxx method)
• prefer lazy val for non-parametrized coders
• Add new Sparkey coders required for testing
• Move implicit coder fallback into com.spotify.scio.coders.kyro

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (964aad7) 63.06% compared to head (b1a9946) 62.56%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
...om/spotify/scio/coders/instances/ScalaCoders.scala	78.94%	4 Missing ⚠️
...spotify/scio/coders/instances/AlgebirdCoders.scala	60.00%	2 Missing ⚠️
...tify/scio/coders/extra/sparkey/SparkeyCoders.scala	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5199      +/-   ##
==========================================
- Coverage   63.06%   62.56%   -0.51%     
==========================================
  Files         300      301       +1     
  Lines       10947    10807     -140     
  Branches      740      764      +24     
==========================================
- Hits         6904     6761     -143     
- Misses       4043     4046       +3     

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[clairemcginty]

it is too bad that the SMB read/transform APIs now require Coders just to execute the isTest path :( But I can't see a way around that.