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

[MateuszKubuszok]

TODO:

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

[codecov[bot]]

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). Report is 90 commits behind head on master.

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

Additional details and impacted files

```diff @@ 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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