[Elixir] Deserialize responses based on status code

Created by: mrmstn

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh, ./bin/security/{LANG}-petstore.sh and ./bin/openapi3/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

First of all, sorry if I might made some chaos. I branched this PR from the great work of @yknx4. I'm hoping the change history will look a little bit better after PR https://github.com/OpenAPITools/openapi-generator/pull/2326 was merged.

With this PR the generated elixir client will be able deserialize a response based on the given statuscodes including the ability to match on a default response in case none of the give codes matches (https://swagger.io/docs/specification/describing-responses/)

This PR stops descriptions and doc parts from beeing escaped by mustache and will pin poison to ~> 3.0.0 since poison 4.0.0 doesn't seems to work right now (I haven't investigated anything)

based on https://github.com/OpenAPITools/openapi-generator/pull/2326 closes https://github.com/OpenAPITools/openapi-generator/issues/2334