[C][Client] Eliminate compiler warnings

[ityuhui]

If we call apiClient_invoke with some const char * as parameters of the function, ( the complete source code is https://github.com/kubernetes-client/c/blob/master/kubernetes/src/generic.c) then there are some warnings while compiling the C client library.

src/generic.c:57:38: warning: passing argument 2 of ‘apiClient_invoke’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
     apiClient_invoke(client->client, path, queryParameters, headerParameters, formParameters, headerType, contentType, body, method);
                                      ^~~~
In file included from src/generic.c:1:0:
src/../include/apiClient.h:43:6: note: expected ‘char *’ but argument is of type ‘const char *’
 void apiClient_invoke(apiClient_t *apiClient,char* operationParameter, list_t *queryParameters, list_t *headerParameters, list_t *formParameters,list_t *headerType,list_t *contentType, char *bodyParameters, char *requestType);

This PR eliminates these compiler warnings.

@wing328 @zhemant @michelealbano

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[zhemant]

Thank you for the PR, LGTM!

[wing328]

Tested locally and the result is good:

[ 73%] Building C object CMakeFiles/openapi_petstore.dir/model/user.c.o
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
[ 80%] Building C object CMakeFiles/openapi_petstore.dir/api/PetAPI.c.o
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
[ 86%] Building C object CMakeFiles/openapi_petstore.dir/api/StoreAPI.c.o
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
[ 93%] Building C object CMakeFiles/openapi_petstore.dir/api/UserAPI.c.o
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
[100%] Linking C shared library libopenapi_petstore.so
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
[100%] Built target openapi_petstore
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)