`apispec` ignores `oneOf` from `marshmallow-oneofschema`

Nobody has claimed this yet.

I need `oneOf` in my schema:

```py
class AerthlingsShopOffer(OneOfSchema):
"""Polymorphic offer type discriminated by the 'type' field."""
type_field = "type"
type_schemas = {
"character": AerthlingsCharacterOffer,
"mutation_point": AerthlingsMpOffer,
}
```

Marshmallow doesn't seem to support this. Fortunately, there is an extension, [`marshmallow-oneofschema`](https://github.com/marshmallow-code/marshmallow-oneofschema). Unfortunately, when presented with the class above, `apispec` silently ignores it, producing

```py
"AerthlingsShopOffer": {
"type": "object",
"properties": {}
},
```

and omitting `AerthlingsCharacterOffer` and `AerthlingsMpOffer` entirely.

I have a fix for this, but I think it requires discussion, and I have not yet written a unit test for it. If you think this is worth doing, I can complete the work and submit a PR. If not, that is no problem for me; I can continue to use my forked `apispec` in my own project.

No source file or test path is named; start by locating the marshmallow-oneofschema handling in apispec and reading the relevant schema-generation tests. Reproduce the AerthlingsShopOffer example, then add regression coverage confirming that oneOf and both concrete offer schemas are emitted.

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