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.

{
    using namespace Poco::Net;
    if (exception_code == ErrorCodes::REQUIRED_PASSWORD)
    {
        return HTTPResponse::HTTP_UNAUTHORIZED;
    }
    else if (exception_code == ErrorCodes::UNKNOWN_USER ||
             exception_code == ErrorCodes::WRONG_PASSWORD ||
             exception_code == ErrorCodes::AUTHENTICATION_FAILED)
    {
        return HTTPResponse::HTTP_FORBIDDEN;
    }
    else if (exception_code == ErrorCodes::CANNOT_PARSE_TEXT ||
             exception_code == ErrorCodes::CANNOT_PARSE_ESCAPE_SEQUENCE ||
             exception_code == ErrorCodes::CANNOT_PARSE_QUOTED_STRING ||
             exception_code == ErrorCodes::CANNOT_PARSE_DATE ||
             exception_code == ErrorCodes::CANNOT_PARSE_DATETIME ||
             exception_code == ErrorCodes::CANNOT_PARSE_NUMBER ||
             exception_code == ErrorCodes::CANNOT_PARSE_INPUT_ASSERTION_FAILED ||
             exception_code == ErrorCodes::UNKNOWN_ELEMENT_IN_AST ||
             exception_code == ErrorCodes::UNKNOWN_TYPE_OF_AST_NODE ||
             exception_code == ErrorCodes::TOO_DEEP_AST ||
             exception_code == ErrorCodes::TOO_BIG_AST ||
             exception_code == ErrorCodes::UNEXPECTED_AST_STRUCTURE ||