Multiple bug fixes with proper attribute inheritance

[n0tl3ss]

This code change fixes issue with inheritance of "description" and "nullable" attributes. The fields annotated with @Schema should always consider their "description" and "nullable" arguments. The "required" argument should be only displayed in the list of the required properties of the DTO.

Fix #659

[swaechter]

@n0tl3ss I just tested your fix with Micronaut 3.3.4 and Micronaut OpenAPI 4.0.1 Snapshot. A lot of our issues were fixed. A big thank you!

I found one issue regarding the nullability of a type. The new generated code looks like this (Attributes like protocol can be null when email sending is disabled):

    ReadEmailOutputLocationDto:
      required:
      - protocol
      type: object
      properties:
        protocol:
          allOf:
          - $ref: '#/components/schemas/EmailSendProtocolDto'
          - description: Protocol used for the connection
      description: Schema that represents an existing email output location
    ReadEmailSettingsDto:
      required:
      - active
      - hostname
      - plaintextPassword
      - port
      - protocol
      - senderEmail
      - username
      type: object
      properties:
        active:
          type: boolean
          description: "Flag that indicates whether the email sending is active or\
            \ not. If set to false, all other values are null"
        hostname:
          type: string
          description: Hostname or IP of the email server or null if email sending
            is disabled
          nullable: true
        port:
          type: integer
          description: Port of the email server or null if email sending is disabled
          format: int32
          nullable: true
        protocol:
          allOf:
          - $ref: '#/components/schemas/EmailSendProtocolDto'
          - description: Protocol used for the connection or null if email sending
              is disabled
            nullable: true
        username:
          type: string
          description: Email username to login or null if email sending is disabled
          nullable: true
        plaintextPassword:
          type: string
          description: Plaintext password for the email user to login in or null if
            email sending is disabled
          nullable: true
        senderEmail:
          type: string
          description: Sender email address that is used to send emails or null if
            email sending is disabled
          nullable: true
      description: Schema that represents the current email settings

Problematic is the location of the nullable:

    ReadEmailSettingsDto:
      required:
        SNIPPED
      type: object
      properties:
        SNIPPED OTHER PROPERTIES
        protocol:
          allOf:
          - $ref: '#/components/schemas/EmailSendProtocolDto'
          - description: Protocol used for the connection or null if email sending
              is disabled
            nullable: true <------------------------------------- Should not be down here

According to https://stackoverflow.com/questions/40920441/how-to-specify-a-property-can-be-null-or-a-reference-with-swagger the null should be at before the allOf, so like this:

    ReadEmailSettingsDto:
      required:
        SNIPPED
      type: object
      properties:
        SNIPPED OTHER PROPERTIES
        protocol:
          nullable: true <------------------------------------- Moved up here
          allOf:
          - $ref: '#/components/schemas/EmailSendProtocolDto'
          - description: Protocol used for the connection or null if email sending
              is disabled

Moving all nullable=true used in combination with allOf in our code fixes the OpenAPI generation.