feat: API schema

[Sander0542]

Currently, the API schema has some missing properties. This PR adds the missing one so that we have a more complete API schema.

Closes #4356

Related to https://github.com/Sander0542/terraform-provider-nginxproxymanager/issues/240

[jc21]

Latest CI Error:

$.paths['/users'].post.responses['201'].content['application/json'].examples['default']
additional properties 'modified_on', 'id', 'created_on', 'user_id' not allowed

This linting is done with vacuum using the rules defined here.

[Sander0542]

Thanks for letting me know! I'll try to push a fix later today