[C][Client] Treat the data type "byteArray" same as "string"

[ityuhui]

If the below data type exists in a OpenAPI spec,

        "data": {
          "additionalProperties": {
            "format": "byte",
            "type": "string"
          },

OpenAPI-Generator (the function isByteArraySchema in ModelUtils.java) will regard the data as a byteArray other than a string

But the model template (model-body.mustache) of CLibcurl generator cannot recognize the data type byteArray, so data will be ignored. This will introduces some problems. e.g. kubernetes-client/c#66

This PR adds the support for the data type byteArray to model-body.mustache, treats it same as the data type "string". (Actually, I copied the code from {{#isString}}...{{/isString}} ). And now the "data" can be generated.

Hi @wing328 @zhemant @michelealbano

Could you please review this PR ? Thanks.

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[zhemant]

@ityuhui Thank you for the PR. I have the following suggestion to make handling of bytearray correct/uniform.

1. Could you please also add byteArray handling in convertToJson function as well?
   https://github.com/OpenAPITools/openapi-generator/blob/ae93625a1b27efaff9904c1d87cb8008661a9ddd/modules/openapi-generator/src/main/resources/C-libcurl/model-body.mustache#L475-L497
   So we handle both cases, parse from json and convert to json.
2. In model mustache, I saw we were handling byte as number. So all the places in the template are as per numeric values which is clearly wrong as it should be string format as given here: https://swagger.io/docs/specification/data-models/data-types/
   Would it be possible for you to update those in this PR as well?

Thank You!

[ityuhui]

Thank you @zhemant

It's very helpful! I have addressed all your comments.

[zhemant]

Thank you for the fixes. LGTM