I want to add a `predefined_query_handler` to serve a single JSON from my query.
For example

```xml
<rule>
      <url>/my/url</url>
      <methods>GET</methods>
      <handler>
        <type>predefined_query_handler</type>
        <query>select '{"my": "json"}' format JSONAsString</query>
      </handler>
    </rule>
```

I can use `RawBLOB` but it sets up Content-Type 'text/plain' instead of 'application/json'.
I think JSONAsString should also work with output.

extern const int CANNOT_PARSE_INPUT_ASSERTION_FAILED;
    extern const int CANNOT_OPEN_FILE;
    extern const int CANNOT_COMPILE_REGEXP;
    extern const int UNKNOWN_ELEMENT_IN_AST;
    extern const int UNKNOWN_TYPE_OF_AST_NODE;
    extern const int TOO_DEEP_AST;
    extern const int TOO_BIG_AST;
    extern const int UNEXPECTED_AST_STRUCTURE;
    extern const int SYNTAX_ERROR;
    extern const int INCORRECT_DATA;
    extern const int TYPE_MISMATCH;
    extern const int UNKNOWN_TABLE;
    extern const int UNKNOWN_FUNCTION;
    extern const int UNKNOWN_IDENTIFIER;
    extern const int UNKNOWN_TYPE;
    extern const int UNKNOWN_STORAGE;
    extern const int UNKNOWN_DATABASE;
    extern const int UNKNOWN_SETTING;
    extern const int UNKNOWN_DIRECTION_OF_SORTING;
    extern const int UNKNOWN_AGGREGATE_FUNCTION;
    extern const int UNKNOWN_FORMAT;
    extern const int UNKNOWN_DATABASE_ENGINE;
    extern const int UNKNOWN_TYPE_OF_QUERY;
    extern const int NO_ELEMENTS_IN_CONFIG;
    extern const int QUERY_IS_TOO_LARGE;
    extern const int NOT_IMPLEMENTED;
    extern const int SOCKET_TIMEOUT;
    extern const int UNKNOWN_USER;
    extern const int WRONG_PASSWORD;
    extern const int REQUIRED_PASSWORD;