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

[cliffano]

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.

[wing328]

@cliffano the change looks good to me. Do you mind adding the plugin to the following Java pom.mustache as well?

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

(Starting from 2.2.0, we've changed the default Java API client from using Jersey 1.x to using okhttp-gson instead)

[wing328]

@cliffano PR merged. When you've time, please update the pom.xml for other Java API client.

[cliffano]

Sorry, was away yesterday.
I've added Maven Javadoc plugin to the other Java clients in #3503 .