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

Created by: 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. https://github.com/kubernetes-client/c/issues/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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