Add Kotlin code generation

[alexander-yevsyukov]

This PR adds Kotlin code generation for proto types. It also updates ProtoTaskExtensions.kt for adding file-wide suppression of deprecations to the generated Kotlin code.

The Protobuf version was rolled back because 3.21.7 does not add visibility modifiers to the generated code. See my comments for this PR for the details on the issue.

[alexander-yevsyukov]

@armiol, @dmdashenkov, it works, but I had to reduce the explicit API level to warnings. There are many of them.

And I still don't know how to suppress them in a good way.

[codecov]

Codecov Report

Merging #747 (6eccd0d) into master (a1a4b45) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #747   +/-   ##
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  Coverage     74.67%   74.67%           
  Complexity     1352     1352           
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  Files           198      198           
  Lines          4786     4786           
  Branches        360      360           
=========================================
  Hits           3574     3574           
  Misses         1085     1085           
  Partials        127      127           

[alexander-yevsyukov]

Protofuf guys say they've fixed it.

[alexander-yevsyukov]

Rolling back to 3.19.6 solves the problem.

I'm a bit puzzled, the fix was 26 days ago. 3.21.7 was released 13 days ago. The merge was into main. The fix should have been included into the release. But I don't see the mentioning in the release notes.