Migrate `java` module to Kotlin

[alexander-yevsyukov]

The controversial title of this PR means the migration of the module responsible for validation code generation written for Java and written on Java.

Some of the code in this module was written in Java and the rest of the code was in Kotlin. Working in the Java part of the code was awkward in particular when dealing with Kotlin extension properties or functions. So, we decided to migrate the codegen parts to Kotlin. This PR starts this process.

Apart from obvious conversion things the following items deserve your attention.

Notable changes

• The code related to InsertionPoints was consolidated under the point subpackage.
• The SetOnceValidationRenderer was moved under the setonce subpackage.
• Extensions for PrimitiveType are now used from ProtoData API.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 51.00000% with 49 lines in your changes missing coverage. Please review.

Project coverage is 33.67%. Comparing base (2e69582) to head (309288e). Report is 23 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #151 +/- ## ============================================ + Coverage 33.51% 33.67% +0.16% + Complexity 356 347 -9 ============================================ Files 134 133 -1 Lines 2709 2693 -16 Branches 220 219 -1 ============================================ - Hits 908 907 -1 + Misses 1739 1723 -16 - Partials 62 63 +1 ```