Tighten extension model

It seems that Arazzo's extension model is a copy of OpenAPI:

• https://spec.openapis.org/arazzo/latest.html#specification-extensions

It always seemed like OpenAPI's extension model should be tightened, and maybe at least for Arazzo we can still do it? Here are two things that I think should go into almost every extension model that tries to be both open and safe:

• Extensions must always be safe to ignore, i.e. when interpreting or processing a description using an extension then it has to be safe to ignore the extension.
• Extensions are not allow to alter the semantics for anything other than their own scope, i.e. they cannot change any semantics of the base specification and they also cannot change the semantics of other extensions.

In the end, I would argue that we're simply applying the principles of good API design to our own spec.

Start with Arazzo's specification-extensions section and the linked OpenAPI specification-extensions section. Review the proposed safety and scope constraints alongside the 12-comment thread; done means the extension rules and resulting specification change have been explicitly decided and documented.