[BUG] Using custom grant types doesn't work

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)? The online validator is also broken
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

According to the oauth2 spec the list of grant types are non-exhaustive and you can add your own grant types.

https://datatracker.ietf.org/doc/html/rfc6749#section-8.3

However, when I supply our custom grant type the tool thinks the openapi file is invalid:

-attribute components.securitySchemes.OAuth2.<redacted> is unexpected

openapi-generator version

<openapi.generator.maven.plugin.version>5.3.0</openapi.generator.maven.plugin.version>

OpenAPI declaration file content or url

components:
  securitySchemes:
    OAuth2:
      type: oauth2
      flows:
        <redacted>:

Steps to reproduce

• Run generator with above openapi yaml.

Suggest a fix

• Allow custom grant types

Workaround

I can work around it right now by disabling the validator, but it would be greater if I could disable only this check.

Start by reproducing the validation failure with the provided OpenAPI YAML and inspect how components.securitySchemes OAuth2 flows are validated. Confirm that a non-standard grant type is accepted without disabling all validation, then add or update a focused regression test for the custom grant type case.