Openapi nested objects properties

[AdamiecRadek]

This PR adds a generation of property types for nested objects inside responses during openapi generation.
Previously if the response contained nested objects, it was marked simply as object, and no additional params were generated. Now OpenAPISpecWriter.php will iterate through its properties and generate specs for them.

Previously

'201':
  description: ''
  content:
    application/json:
      schema:
        type: object
        #example: ...
        properties:
          this:
            type: string
            example: shouldn't be ignored
          and this:
            type: string
            example: too
            description: Parameter description, ha!
          sub level 0:
            type: object
            example:
              sub level 1 key 1: sl0_sl1k1
              sub level 1 key 2:
              - sub level 2 key 1: sl0_sl1k2_sl2k1
                sub level 2 key 2:
                  sub level 3 key 1: sl0_sl1k2_sl2k2_sl3k1
              sub level 1 key 3:
                sub level 2 key 1: sl0_sl1k3_sl2k2
                sub level 2 key 2:
                  sub level 3 key 1: sl0_sl1k3_sl2k2_sl3k1
                  sub level 3 key null:
                  sub level 3 key integer: 99
'204':
  description: Successfully updated.

Now

'201':
  description: ''
  content:
    application/json:
      schema:
        type: object
        #example: ...
        properties:
          this:
            type: string
            example: shouldn't be ignored
          and this:
            type: string
            example: too
            description: Parameter description, ha!
          sub level 0:
            type: object
            properties:
              sub level 1 key 1:
                type: string
                example: sl0_sl1k1
              sub level 1 key 2:
                type: array
                example:
                - sub level 2 key 1: sl0_sl1k2_sl2k1
                  sub level 2 key 2:
                    sub level 3 key 1: sl0_sl1k2_sl2k2_sl3k1
                items:
                  type: object
              sub level 1 key 3:
                type: object
                properties:
                  sub level 2 key 1:
                    type: string
                    example: sl0_sl1k3_sl2k2
                    description: This is description of nested object
                  sub level 2 key 2:
                    type: object
                    properties:
                      sub level 3 key 1:
                        type: string
                        example: sl0_sl1k3_sl2k2_sl3k1
                      sub level 3 key null:
                        type: string
                        example:
                      sub level 3 key integer:
                        type: integer
                        example: 99
'204':
  description: Successfully updated.