[Bug][cpp-restsdk]Call extract_string() will not get correct string when media type is not "application/json"

Bug Report Checklist

• [ ✓ ] Have you provided a full/minimal spec to reproduce the issue?
• [ ✓ ] Have you validated the input using an OpenAPI validator ([example](https://apidevtools.org/swagger-parser/online/))?
• [ ✓ ] 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?
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Description

1.CPP-RESTSDK will extract string with default input parm : ignore_content_type = false https://github.com/microsoft/cpprestsdk/blob/master/Release/include/cpprest/http_msg.h#L613-L619
2.When a json has a customed content type , the content type validation will cause the extract string get incorrect value.

3.Additional,As https://en.wikipedia.org/wiki/Media_type says , there is common content type like "application/json" and also other customed content type , which only need to follow the standards.

Why this will cause incorrect value , jump to the code ,we see :

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/cpp-rest-sdk-client/api-source.mustache#L309

openapi-generator version

master , 5.4.0

Suggest a fix

For 2. ,suggest hard code the extract_string(true)

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/cpp-rest-sdk-client/api-source.mustache#L309

return localVarResponse.extract_string(true);

Start at modules/openapi-generator/src/main/resources/cpp-rest-sdk-client/api-source.mustache around line 309 and review how extract_string() handles non-application/json media types. Update the generated call as suggested, then verify that generated C++ REST SDK clients extract the expected string for custom media types.