[Java] Add maven-javadoc-plugin in pom.xml of generated Java client

[wing328]

Description

This plugin would allow the generation of Javadoc from the generated Java client using mvn javadoc:javadoc, making it easier to document the Java client API.

@cliffano has updated it for jersey1 Java API client via #3482

If anyone has time to update pom.xml for other Java API clients (listed below), please reply to let us know.

https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/Java/libraries/feign/pom.mustache
https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/Java/libraries/jersey2/pom.mustache
https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/Java/libraries/retrofit/pom.mustache
https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/Java/libraries/retrofit2/pom.mustache
https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/Java/libraries/okhttp-gson/pom.mustache

[wing328]

PR merged. Thanks for the contribution from @cliffano