[Java][jaxrs] Add getter to ApiException templates

Created by: Xendar

Fixes #6830 (closed)

Basically a few ApiException templates for Java servers were missing a getter for the code field.
Before the fix the generated class was:

package org.openapitools.api;

@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaResteasyServerCodegen", date = "2020-07-01T16:12:55.083+02:00[Europe/Paris]")
public class ApiException extends Exception{
    private int code;
    public ApiException (int code, String msg) {
        super(msg);
        this.code = code;
    }
}

With the fix it becomes:

package org.openapitools.api;

@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaResteasyServerCodegen", date = "2020-08-06T22:46:58.073+02:00[Europe/Paris]")
public class ApiException extends Exception {

    private int code;

    public ApiException(int code, String msg) {
        super(msg);
        this.code = code;
    }

    public int getCode() {
        return code;
    }

}

Validation

I compiled the cli jar and generated the new class with the same command as in the issue description, then compiled the generated code.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

Technical committee: @bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @bkabrda