[core][Python] rest client: handle multiple response types

Created by: rienafairefr

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

In OAS 3 it's possible to return multiple content-type, and for now the rest clients in the different languages don't support that, e.g. problems or related PRs: #206 (closed) #1367 #440 (closed) I've added returnType returnType field to the CodegenResponse class, this way we can get a mapping between the response code and the expected return type of that response, and I've implemented that logic in the Python client, through a response_types parameter dictionary. The Accept header is modified to include all the cases that the client expects, not just application/json. The REST response is UTF-8 decoded if needed (not decoded in case of a file type), and deserialized according to the response type expected for the HTTP status code that is encountered.

Tested working on a API endpoint that returns a binary file on HTTP 200, and can also send a JSON with an error message for HTTP 401/403/500.