[BUG] [haskell] oneOF is not handled correctly

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Trying to create server stubs in haskell for 5G core functions.
When the yaml file uses oneOf for a object property, the created haskell code in the corresponding Types.hs file cannot be parsed by GHC.
For example for yaml file https://github.com/jdegre/5GC_APIs/blob/master/TS29509_Nausf_UEAuthentication.yaml line 170 results in:
, uEAuthenticationCtx5gAuthData :: OneOf<Av5gAka,string> -- ^

openapi-generator version

$ docker run --rm openapitools/openapi-generator-cli version
4.0.0-SNAPSHOT

OpenAPI declaration file content or url

https://github.com/jdegre/5GC_APIs/blob/master/TS29509_Nausf_UEAuthentication.yaml

Command line used for generation

docker run --rm -v $PWD:/local openapitools/openapi-generator-cli generate -i /local/5GC_APIs/TS29509_Nausf_UEAuthentication.yaml -g haskell -o /local/haskell/genapi/ausf_auth/

Steps to reproduce

$ grep -rni oneof haskell/genapi/ausf_auth/
haskell/genapi/ausf_auth/lib/AUSF/Types.hs:279: , uEAuthenticationCtx5gAuthData :: OneOf<Av5gAka,string> -- ^

Related issues/PRs

#15 ???

Suggest a fix

Reproduce the issue with the linked TS29509_Nausf_UEAuthentication.yaml using the Docker generation command, then inspect haskell/genapi/ausf_auth/lib/AUSF/Types.hs around line 279. Confirm that the oneOf property is emitted as valid Haskell and that the generated Types.hs parses successfully with GHC.