[BUG][All generators] Specification extension for info object is not exposed

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

According to the documentation the info object can be extended with Specification Extensions / Vendor extensions. However it does not work and generators does not expose that information to the templates.

It works for example in OpenApi object and Opeartion object but not in Info object

openapi-generator version

4.2.0-SNAPSHOT

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Info service
  x-api-prefix: UFO
  version: 1.1.0
servers:
  - url: http://localhost:8080/v1
    description: PROD
paths:
  /info:
    get:
      summary: Get some info
      operationId: getInfo
      responses:
        200:
          description: OK

and mustach template (api.mustache):
{{this}}

Command line used for generation

java -jar ../../openapi-generator/modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g java -i openapi/info-service-v1.yaml -o serviceAPIs/Info -t ./openapi/template/

(build from master branch)

Same result with different generator:

npx openapi-generator generate -g typescript-axios -i openapi/info-service-v1.yaml -o serviceAPIs/Info -t ./openapi/template

Steps to reproduce

Just try to generate code, for Java generator look into file:
serviceAPIs/Notification/src/main/java/org/openapitools/client/api/DefaultApi.java
and try to find UFO

For npm look of UFO in api.ts file

Related issues/PRs

Suggest a fix

Fix should expose extensions to the mustache templates

Run the provided Java or typescript-axios generation command using the YAML declaration and custom api.mustache template. Compare the template output with the generated DefaultApi.java or api.ts, then trace how the Info object reaches templates. Done means the x-api-prefix value UFO is exposed and appears in generated output.