[plugins] `mustExist` for custom rules does not give an error for additional properties

Is your feature request related to a problem? Please describe. following the example for Plugins

I successfully created an example to test required properties in a Response object but I was not able to figure out how to evaluate additional properties in the schema object. I want to ensure the mustExist array are the only properties defined in the schema.

Describe the solution you’d like I would like to know how to use mustExist in the custom rule to enforce a specific schema.

[RomanHotsiy]

@jeremyfiel your code looks valid!

I would just improve location arg from the last report:

location: location.child(['properties', key]).key(),

Can this issue be resolved?

[jeremyfiel]

thanks Roman.