Implement ValidatingBuilder instead of McJava

[alexander-yevsyukov]

This PR makes the Validation library make Builder classes implement ValidatingBuilder instead of McJava.

Currently, McJava does that via the io.spine.tools.mc.java.protoc.message.BuilderGen and associated classes which work in the protoc plugin of McJava.

This PR introduces ImplementValidatingBuilder renderer which makes Builder classes implement ValidatingBuilder. Adding the code is done only if the Builder class does not implement the interface already. Such a case is possible when older McJava is used together with new Validation. Once McJava migrates to newer Validation, the check would be always false.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 46 lines in your changes missing coverage. Please review.

Project coverage is 37.82%. Comparing base (7129197) to head (6a5ee6b).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #127      +/-   ##
============================================
- Coverage     38.32%   37.82%   -0.51%     
  Complexity      353      353              
============================================
  Files           125      127       +2     
  Lines          2338     2369      +31     
  Branches        197      200       +3     
============================================
  Hits            896      896              
- Misses         1380     1411      +31     
  Partials         62       62              

[armiol]

@alexander-yevsyukov LGTM with a minor comment.