[kotlin-client] Fixing bug in Kotlin Client with BigDecimal default value

[sjoblomj]

Fixes #10866.

When a property is of type number and has a default value, the Kotlin Client will generate code like val bar: java.math.BigDecimal? = 0, which will not compile, since an Int cannot be assigned to BigDecimal. This PR fixes that bug.

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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

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@jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m

[wing328]

Tested locally and the result is good:

gradle build
Starting a Gradle Daemon (subsequent builds will be faster)

> Task :compileKotlin
w: /Users/williamcheng/Code/openapi-generator/samples/client/petstore/kotlin-bigdecimal-default/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt: (115, 6): This class can only be used with the compiler argument '-Xopt-in=kotlin.RequiresOptIn'

BUILD SUCCESSFUL in 1m 12s
3 actionable tasks: 3 executed