[typescript-fetch] Support fileNaming option (#18283)

[En0s0und]

Description

Add new option "fileNaming" for the typescript-fetch generator with following eligible values :

• PascalCase (default)
• camelCase
• kebab-case

That option already exists in typescript-angular and typescript-nestjs generators.

fixes: https://github.com/OpenAPITools/openapi-generator/issues/18283 Original PR for typescript-angular generator: https://github.com/OpenAPITools/openapi-generator/issues/727

To validate

You can save the following specs in a file called test-spec.yaml

openapi: 3.0.0
info:
  version: 1.0.0
  title: Example for fileNaming option
paths:
  /pet:
    get:
      tags:
        - pet-controller
      operationId: getPet
      responses:
        '200':
          description: successful operation
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Pet'
components:
  schemas:
    PetCategory:
      properties:
        id:
          type: integer
    Pet:
      properties:
        id:
          type: integer
        category:
          $ref: '#/components/schemas/PetCategory'

Then run following commands

java -jar openapi-generator-cli.jar generate -i test-spec.yaml -g typescript-fetch -o in-PascalCase

java -jar openapi-generator-cli.jar generate -i test-spec.yaml -g typescript-fetch -o in-kebab-case --additional-properties "fileNaming=kebab-case"

java -jar openapi-generator-cli.jar generate -i test-spec.yaml -g typescript-fetch -o in-camelCase --additional-properties "fileNaming=camelCase"

File names must have the wanted format for each case and those file names must be used in imports.

@TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @topce (2018/10) @akehir (2019/07) @petejohansonxo (2019/11) @amakhrov (2020/02) @davidgamero (2022/03) @mkusaka (2022/04)

PR checklist

• [x] Read the contribution guidelines.
• [x] Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• [x] Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH) Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• [x] File the PR against the correct branch: master (upcoming 7.1.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• [x] If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[En0s0und]

Thanks for the code review ! Constants had been introduced and conflict resolved.