[Kotlin client] Improve JSON parser

[karismann]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.
  @jimschubert (2017/09) ❤️, @dr4ke616 (2018/08) @Zomzog (2019/04)

Description of the PR

• add UUID, LocalDateTime, LocalDate adapter for JSON serialize/deserialize (fix [BUG][KOTLIN] No JsonAdapter for LocalDateTime in Moshi serializer #1271)
• map date to LocalDate type (fix [bug][Kotlin] Default typeMapping for 'date' should be LocalDate #2063)
• fix import in model for threetenbp date (fix [BUG][KOTLIN] Wrong import for fields with "format: date" #1647)

[jimschubert]

This looks good. The typeMapping from LocalDateTime to LocalDate may be seen as a breaking change, but I'd consider this a bug. I thought we'd fixed this LocalDate issue in the Kotlin base, but I looked and it was only applied in KotlinSpringServerCodegen.

If users find this change breaks existing clients, they may regenerate with --type-mappings date=java.time.LocalDateTime for the previous behavior.

[jimschubert]

@karismann this looks good. Is it ready to merge?

[karismann]

@jimschubert yes, thanks for the review!