[java] Support for number enum

[jmini]

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Description of the PR

Fixes #447

The type number is mapped to java BigDecimal. This PR fixes the compilation issues that appears when enum values are defined.

Tested with:

openapi: 3.0.1
info:
  title: issue 447
  version: '1.0'
servers:
  - url: 'http://localhost:9999/'
paths:
  /ping:
    get:
      operationId: pingGet
      responses:
        '201':
          description: OK
components:
  schemas:
    NumberEnum:
      type: number
      enum:
        - 1.0
        - 2.5
        - 3.0

    SomeObject:
      type: object
      properties:
        aNum:
          type: number
          enum:
            - 5.0
            - 6.5
            - 7.0

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[wing328]

LGTM