MXS-1779 Add initial support for classification URL

This commit introduces the plumbing support for obtaining
classification information of a statement using the REST-API.
It introduces a URL like

    /v1/maxscale/query_classifier/classify?sql=SELECT+1

that in the response will return a JSON object with the
information. Subsequent commits will provide the actual
information.

include/maxscale/config.h

@@ -110,6 +110,7 @@ extern const char CN_AUTH_READ_TIMEOUT[];
 extern const char CN_AUTH_WRITE_TIMEOUT[];
 extern const char CN_AUTO[];
 extern const char CN_CACHE_SIZE[];
+extern const char CN_CLASSIFY[];
 extern const char CN_CONNECTION_TIMEOUT[];
 extern const char CN_DUMP_LAST_STATEMENTS[];
 extern const char CN_DATA[];