Handle OpenAPI 3.1 array type specifier in SerDes

[Pijukatel]

Handle OpenAPI 3.1 array type specifier in SerDes.
Add test.

Example type specifiers: [string, 'null'], [array, 'null']

Example schema:

paths:
  /nullable_dates:
    get:
      responses:
        200:
          description: 'Get nullable array of nullable dates'
          content:
            application/json:
              schema:
                # 3.1 syntax nullable array
                type: [array, 'null']
                items:
                  type: object
                  properties:
                    createdAt:
                      # 3.1 syntax nullable item
                      type: [string, 'null']
                      format: date-time

Running validator on response according to this schema would previously throw this warning:
/response/1/createdAt must be string,null

Related issue: #1177

[Pijukatel]

Hello @cdimascio, we are using the validator in our project, and we see a lot of false positives related to the OpenAPI 3.1 array type syntax.

I am not familiar with the validator implementation, so this is my naive attempt to fix it. Please let me know if you think it is going in the right direction. Thanks.