[feat]: support for x-additionalPropertiesName when generating property names

[burnpiro]

At this moment when someone is using additionalProperties it generates property1 and property2 as key names. Sampler could use the x-additionalPropertiesName property to generate custom property names:

{
   additionalProperties: {
      type: 'string', 
      'x-additionalPropertiesName': 'attribute-name'
   }
}

Generates:

{
      'property1': 'string',
      'property2': 'string'
}

and the output should look like this:

{
      'attribute-name1': 'string',
      'attribute-name2': 'string'
}

Default property name should remain the same when no x-additionalPropertiesName is defined.

This is not a standard property of additionalProperties but Redocly supports it in the schema definition. I'll make a PR to change it.