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.

IServer & server_,
    const NameSet & receive_params_,
    const std::string & predefined_query_,
    const CompiledRegexPtr & url_regex_,
    const std::unordered_map<String, CompiledRegexPtr> & header_name_with_regex_)
    : HTTPHandler(server_, "PredefinedQueryHandler")
    , receive_params(receive_params_)
    , predefined_query(predefined_query_)
    , url_regex(url_regex_)
    , header_name_with_capture_regex(header_name_with_regex_)
{
}
bool PredefinedQueryHandler::customizeQueryParam(ContextMutablePtr context, const std::string & key, const std::string & value)
{
    if (receive_params.contains(key))
    {
        context->setQueryParameter(key, value);
        return true;
    }
    return false;
}
void PredefinedQueryHandler::customizeContext(HTTPServerRequest & request, ContextMutablePtr context)