Support Path Item references (OpenAPI 3.1 only)

A file that works in openapi generator does not work in openapi-generator. The section that causes the error is:
```yaml
paths:
/documents:
$ref: './paths/documents.yaml#/documents'
/documents/{document-ident}:
$ref: './paths/documents.yaml#/documents-detail'
```
in the root file.

Error:
```
error: Inconsistency encountered when parsing `Vendor Extension` under the `/documents` path: Found at least one vendor extension property that does not begin with the required 'x-' prefix. Invalid properties: [ $ref ].
```

We always have our paths in separate files for various reasons. This never was a problem before so I think swift-openapi-generator should implement it. ;)

Start by reproducing the supplied OpenAPI 3.1 YAML with path items referenced from a separate file, then trace where the parser treats `$ref` under `/documents` as a vendor extension. Done means valid Path Item references are accepted without the reported error while other invalid vendor extensions remain rejected.

Written by the indexing model from the issue text.