[BUG] extension refs are not supported

Description

The generator ignores OpenAPI extension references ( https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#patterned-objects ) because this conditional has whitelist of references

https://github.com/OpenAPITools/openapi-generator/blob/046db19a8590f3557ae1e04be75792bbeb39cb69/modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java#L279-L287

I'm using the GRPC-gateway to generate openAPI spec for my gRPC apis, and it uses OpenAPI extensions to hold references to the stream response (See this PR https://github.com/grpc-ecosystem/grpc-gateway/pull/850 ) This generator puts x-stream-definitions refs at the root of the openAPI spec. Which gives these warnings

[main] WARN  o.o.codegen.utils.ModelUtils - #/x-stream-definitions/aggregationListAggregationsResponse is not defined
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: #/x-stream-definitions/aggregationListAggregationsResponse

openapi-generator version

4.0.0

OpenAPI declaration file content or url

https://gist.github.com/seanlaff/9a8d88027bd775b5ef31e63abf3fb946

Command line used for generation

docker run --rm -v ${PWD}:/local openapitools/openapi-generator-cli:v4.0.0-beta generate \
    -i /local/test.json \
    -g typescript-axios \
    -o /local/out/ts

Start with the whitelist conditional in modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java at the linked lines, then reproduce the issue with the provided gist and Docker generation command. Compare generation involving the x-stream-definitions references against the reported warnings. Done means extension references are handled without those warnings.