[Elm] Add support for petstore spec

[Qluxzz]

Add support for generating the petstore client.

• Change default request body from Json to any variant of Http.Body, to allow form param body
• Added support for form params, including files
• Fix mapping multiple response types while we only can handle one today
• Fix order of operation with parenthesis
• Fix not exporting needed enum to string methods from Api.Data
• Fix handling of inline enums and enum refs
• Remove whitespace lambda now allows better formatting of templates

I also removed the old generated petstore client in samples/openapi3/petstore/elm, this should probably have been removed in #4891, since there has been no config to generate it since then. That generated client was also generated before the big refactor in that same pull request.

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  @eriktim

[wing328]

@Qluxzz thanks for the PR.

Would it be possible to setup Github workflow to test the Elm Petstore client as part of this PR? 🙏

ref:

• https://github.com/marketplace/actions/setup-elm-environment
• https://github.com/OpenAPITools/openapi-generator/blob/master/.github/workflows/samples-php8.yaml

[Qluxzz]

Definitely, I have added a pipeline that validates that all elm files successfully compiles.

[Qluxzz]

@wing328 the CI failures seem to be related to python-aiohttp and python-nextgen which is nothing I have touched in this PR.

[wing328]

Agreed.

I've just merged it. Thanks for the PR 👍