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openapi: Validate unregistered type, if they can be found
**What this PR does / why we need it**:
Types that are not registered/hard-coded in kubectl won't be validated, even if they could because they are defined in openapi. If they are neither registered nor in openapi, then skip validation.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes nothing
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
References in the openapi are currently completely hidden from the
model, and just passed through as we walk the tree. The problem is that
they can have a different description and more importantly, different
extensions.
Change them to be first-class citizen, and fully part of the model. It
means that visitors have to implement one more function and decide if
something specific should be done with references. Validation is updated
to just completely ignore them and passthrough (like it was done
before).
This allows validation of a yaml/json object against an openapi schema.
A lot more testing would be needed to validate the logic, and also this
is not plumbed in, so it can't be used by kubectl yet.
