[JAVA] Enhance JSON.mustache to be able to handle LocalDateTime fomatted date-time

Description

So far, when trying to replace a OffsetDateTime object with a LocalDateTime (to be able to read from a server which is not following the standard...), i was able to do so by mapping the fields, alas, the generated code seems not to be able to run, as it had no idea how to deserialize a LocalDateTime. Now i am not sure if i just did the mapping wrong... But anyhow, i patched the JSON.mustache file, and since it seems to work nicely.

Swagger-codegen version

I used 2.4.6

Swagger declaration file content or url

https://unitest01.public360online.com/Biz/v2/api/swagger/SI.Data.RPC/SI.Data.RPC/CaseService

Command line used for generation

java -jar ../swagger-codegen-cli-2.4.6.jar generate -i "https://unitest01.public360online.com/Biz/v2/api/swagger/SI.Data.RPC/SI.Data.RPC/CaseService" -l java -c case.json -o public360-case-service

with the case.json lookig lik that:

{
  "groupId": "no.unit.p360.rpc",
  "artifactId": "public360-case-service",
  "artifactVersion": "1.0.0",
  "library": "okhttp-gson",
  "modelPackage": "no.unit.p360.cases.model",
  "invokerPackage" : "no.unit.p360.cases",
  "apiPackage": "no.unit.p360.cases.api",
  "generateSupportingFiles" : true,
  "codegenVersion": "V2",
  "useOas2": true,
  "dateLibrary": "java8",
  "java8": true,
  "importMappings": {
          "GetCasesArgsParameter" : "no.unit.mule.p360.cases.model.GetCasesArgsParameter",
          "java.time.OffsetDateTime" : "java.time.LocalDateTime"
  },
  "typeMappings" : {
          "OffsetDateTime" : "java.time.LocalDateTime"
  }

}

Steps to reproduce

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/issues/7589 is somehow related, but this one tries not to introduce a new mapping

Suggest a fix/enhancement

JSON.zip

Start with the JSON.mustache template and reproduce the issue using the documented swagger-codegen 2.4.6 command and case.json mappings. Compare generated LocalDateTime handling with the attached JSON.zip patch; done means generated Java can deserialize LocalDateTime when OffsetDateTime is mapped as shown.