Users should be able to query configurable vendor extensions via CLI

Description

Refer to https://github.com/OpenAPITools/openapi-generator/wiki/Vendor-Extensions

Generators which support user-configurable vendor extensions should expose these as valuable information to the user via CLI.

openapi-generator version

n/a

Related issues/PRs

n/a

Suggest a fix/enhancement

To reduce the amount of information exposed via config-help, and because vendor extensions aren't generally a means for configuring codegen but for extending output, we may want to consider making these queryable as a subcommand.

Some possible options for CLI usage:

$ cli config-help extensions --generator-name=java-feign
Generator: java-feign
Extensions:
  x-accepts
      A single Accepts value as the Feign API client needs a single value for Accepts header

  x-content-type
      A single "Content-Type" value as the Feign API client needs a single value for Content-Type header

$ cli config-help extensions --generator-name=cwiki
Generator: cwiki
Extensions: none

We may even want to consider exposing a metadata-only command, and moving config-help as a sub-command, but we can worry about shuffling the CLI options around within a minor release. For this feature request, I'd like to get extension information queryable.