Add rule RemovedRequiredProperty

[guillaume-pansier]

Resolves #26

[holm0563]

why would removing a required property be breaking? Any UI would still work without a re-deployment

[guillaume-pansier]

Hello @holm0563, my stake on this is that if your API is exposing a property marked as required, there's a chance that your client are relying on it, and assume it's present. You mention UIs, so I'll take a TS application example, you could have something like this for example:

var myResponse: APIModel = await callAPI();
// -> cannot read property of undefined
const thisMightBreak = myResponse.attributeThatIsMarkedAsRequired.someNestedAttributes
// -> change of behavior
if(myResponse.otherBooleanAttributeMarkedAsRequired) {
  // do something
}

[guillaume-pansier]

Actually, we discussed that a bit in the linked issue, here's the relevant part:

In a Response,

a RemovedRequiredProperty should be always considered as a breaking change error, since the existing clients could consider as an error a missing required property
a AddedRequiredProperty is only an informative message since it should be ignored by the existing clients.

In a Request, it is more subtle:

a RemovedRequiredProperty can be an acceptable (non-breaking) change error as long as the property is still in the schema and the server will ensure a backward compatible behavior.
a AddedRequiredProperty is always a breaking change error since the existing won't provide the property.