[Java] option for the prefix of boolean getters

Created by: jmini

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language. Java: @bbdouglas (2017/07) @JFCote (2017/08) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01)

Description of the PR

This PR adds a new option to all java generators (java client, jaxrs server, spring, …): booleanGetterPrefix

This new option influence the name of the getter generated for a boolean property in a model.

Example values for booleanGetterPrefix:

• is: default value in 3.0.x.
• get: new default value starting with 3.1.0

With following OAS3 (extract):

components:
  schemas:
  schemas:
    Order:
      type: object
      properties:
        id:
          type: integer
          format: int64
        status:
          type: string
          enum:
            - placed
            - approved
            - delivered
        complete:
          type: boolean

Generated code with is value:

  public Boolean isComplete() {
    return complete;
  }

With get value:

  public Boolean getComplete() {
    return complete;
  }

This was also discussed in:

• swagger-api/swagger-codegen#7261
• swagger-api/swagger-codegen#7764
• swagger-api/swagger-codegen#7638
• ...