[BUG][Swift] Generating standalone Swift models should not conform them to JSONEncodable

Created by: stjernegard

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
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?
[Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Swift models when generated without API or supporting files, should not conform to protocols defined only in those supporting files. Currently the swift5 generator outputs models conforming to a JSONEncodable protocol, which isn't defined in the output.

openapi-generator version

The problem was introduced in release v5.4.0 with commit https://github.com/OpenAPITools/openapi-generator/commit/febf49662a4d4fda5dbeed4888a1840db801177d and exists in 6.0.0 and in the current master branch.

OpenAPI declaration file content or url

---
openapi: '3.0.0'
paths:
  /:
    get:
      responses:
        200:
          description: OK
      description: Dummy endpoint

info:
  title: MyModel
  version: '1.0.0'

components:
  schemas:
    mymodel:
      type: object
      properties:
        name:
          type: string

Generation Details

docker run --rm -v "${PWD}:/local" openapitools/openapi-generator-cli:v5.4.0 generate \
    -i /local/mymodel.yaml \
    -g swift5 \
    -o /local/out \
    --global-property=models,modelDocs=false

Steps to reproduce

Save the above YAML file as mymodel.yaml
Run the above docker command

Related issues/PRs

There doesn't seem to be any other reports of the same bug. I tried searching all open and closed issues for JSONEncodable, but found nothing relevant.

Suggest a fix

The protocol conformance could be only added if the protocol itself is also being generated.