Enable defaults for specific type, allow providing default values for type when absent in constructor

[MateuszKubuszok]

TODO:

• define trait DefaultValue[A]
  • provide summoning in internals
• define flag DefaultValueOfType[A]
  • parse config in internals
  • allow setting it with DLS
• use flag and type class in ProductToPoduct rule
• provide DefaultValue for UnknownFieldSet
• tests
  • defaultValueOfType[A] tests
  • DefaultValue[A] tests
  • DefaultValue[UnknownFieldSet] tests
• docs
  • providing custom default values (when constructor is missing them)
  • Protobufs and UnknownFieldSet
  • Scaladoc

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 87.71%. Comparing base (e0a48a2) to head (7bfd263).

Files	Patch %	Lines
...ey/internal/compiletime/ChimneyTypesPlatform.scala	50.00%	5 Missing ⚠️
...io/scalaland/chimney/dsl/TransformerFlagsDsl.scala	50.00%	2 Missing ⚠️
...ey/protobufs/ProtobufsDefaultValuesImplicits.scala	0.00%	1 Missing ⚠️
...ey/internal/compiletime/ChimneyTypesPlatform.scala	90.00%	1 Missing ⚠️
...rmSealedHierarchyToSealedHierarchyRuleModule.scala	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #558      +/-   ##
==========================================
- Coverage   87.84%   87.71%   -0.14%     
==========================================
  Files         148      149       +1     
  Lines        5741     5826      +85     
  Branches      466      475       +9     
==========================================
+ Hits         5043     5110      +67     
- Misses        698      716      +18     

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