[java] Enum in array of array

[jmini]

Fix for #16

---

This PR introduces CodegenProperty.mostInnerItems and CodegenParameter.mostInnerItems that gives always the most inner items type when arrays are contained in arrays.

OAS2 (extract):

definitions:
  SomeObj:
    type: object
    properties:
      order:
        type: array
        items:
          type: array
          items:
            type: string
            enum:
              - val1
              - val2
              - val3

Example in Java:
The CodegenProperty created for order defines the type List<List<OrderEnum>>.

Before this PR, if you want to access the CodegenProperty corresponding to the enum you need to do items.items (because we have an array of array of enum). The problem is that you do not know how many time you need to loop. If you just have a simple array of enum it will be items

In the Java code you often see this pattern:

                while (codegenProperty != null) {
                    codegenProperty = codegenProperty.items;
                }

To find the most inner items CodegenProperty

This PR introduces mostInnerItems that you can call at top level (you should check isContainer) and that gives you the items you are looking for (the one corresponding to OrderEnum in the example)

[jmini]

I could not use the gist provided by @msjoshi in the original issue swagger-api/swagger-codegen#7918 because of #50. So I tested this PR with this OpenAPI version 2 Specification:

swagger: '2.0'
info:
  title: Patrocinium
  description: Patrocinium API
  version: 1.0.0
  termsOfService: 'https://www.patrocinium.com/end-user-licensing-agreement/'
  contact:
    name: Patrocinium
    url: 'https://www.patrocinium.com'
    email: support@patrocinium.com
host: papi.patrocinium.com
basePath: /v1
schemes:
  - https
  - http
consumes:
  - application/json
produces:
  - application/json
paths:
  /db/ActionRole/findAndCountAll:
    post:
      tags:
        - db
      description: Find all instances of ActionRole
      operationId: findActionRole
      parameters:
        - in: body
          name: body
          required: false
          schema:
            $ref: '#/definitions/ActionRoleSearch'
      responses:
        '200':
          description: Successful Operation
        '400':
          description: Bad Request
        '403':
          description: Unauthorized
        '500':
          description: Internal Server Error
definitions:
  ActionRoleSearch:
    type: object
    properties:
      attributes:
        description: A list of the attributes that you want to return.
        type: array
        items:
          type: string
          enum:
            - id
            - actionId
            - roleId
            - createdAt
            - updatedAt
            - deletedAt
      order:
        description: A collection of grouped attribute names and directions.
        type: array
        items:
          type: array
          items:
            type: string
            enum:
              - id
              - actionId
              - roleId
              - createdAt
              - updatedAt
              - deletedAt
              - ASC
              - DESC
      limit:
        description: The number of ActionRole instances to return.
        type: integer
        format: int64
        default: 100
        minimum: 1
        maximum: 1000
      offset:
        description: The number of ActionRole instances to offset the returned rows.
        type: integer
        format: int64
        minimum: 0
        default: 0

[wing328]

Performed some tests and the result is good.