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.

else if (param_could_be_skipped(key))
        {
        }
        else
        {
            
            if (!customizeQueryParam(context, key, value))
                settings_changes.push_back({key, value});
        }
    }
    if (!database.empty())
        context->setCurrentDatabase(database);
    if (!default_format.empty())
        context->setDefaultFormat(default_format);
    
    context->checkSettingsConstraints(settings_changes);
    context->applySettingsChanges(settings_changes);
    
    context->setCurrentQueryId(params.get("query_id", request.get("X-ClickHouse-Query-Id", "")));
    
    
    query_scope.emplace(context);
    
    
    
    const auto & query = getQuery(request, params, context);
    std::unique_ptr<ReadBuffer> in_param = std::make_unique<ReadBufferFromString>(query);
    in = has_external_data ? std::move(in_param) : std::make_unique<ConcatReadBuffer>(*in_param, *in_post_maybe_compressed);