Replace moshi with kotlinx-serialization

[agologan]

This PR extends on #136. There's still work to be so external input is more then welcome:

Changes that should be discussed:

• replaced ZonedDateTime with Instant from kotlinx-datetime - while this logically correct it does reduce all datetime representations to Zulu timezone
• replaced number from BigDecimal to double - there is no official kmp support for BigDecimal which is not great because double is an approximation
• replaced object with JsonObject - while using Json* classes is more type-safe it does modify the API heavily

As you can see most problems are around unsupported types which we could support via the Java classes but would still require some serialization changes maybe via contextual serialization?

The last problematic point is regarding nullability. While deserialization works as expected serialization implicitly omits null values. As far as I can tell these could be added back if the integrator passes in encodeDefaults: true on the JsonConfiguration. The x-nullable vendor extension just states null as a valid value and doesn't imply the client should explicitly send null to define a property as missing so maybe this is not actually a problem.

[codecov[bot]]

Codecov Report

Merging #152 (2b0a77f) into master (b2f2c2f) will decrease coverage by 0.04%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #152      +/-   ##
============================================
- Coverage     71.95%   71.90%   -0.05%     
  Complexity      139      139              
============================================
  Files            11       11              
  Lines           574      573       -1     
  Branches         75       75              
============================================
- Hits            413      412       -1     
  Misses          118      118              
  Partials         43       43              

Impacted Files	Coverage Δ
.../src/main/java/com/yelp/codegen/KotlinGenerator.kt	84.42% <ø> (ø)
...ain/java/com/yelp/codegen/utils/KotlinLangUtils.kt	100.00% <100.00%> (ø)

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